diff --git a/Tests/test_file_apng.py b/Tests/test_file_apng.py
index ee6c867c3ca..27fc0e48294 100644
--- a/Tests/test_file_apng.py
+++ b/Tests/test_file_apng.py
@@ -12,6 +12,7 @@
 # (referenced from https://wiki.mozilla.org/APNG_Specification)
 def test_apng_basic() -> None:
     with Image.open("Tests/images/apng/single_frame.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert not im.is_animated
         assert im.n_frames == 1
         assert im.get_format_mimetype() == "image/apng"
@@ -20,6 +21,7 @@ def test_apng_basic() -> None:
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
     with Image.open("Tests/images/apng/single_frame_default.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert im.is_animated
         assert im.n_frames == 2
         assert im.get_format_mimetype() == "image/apng"
@@ -49,6 +51,7 @@ def test_apng_basic() -> None:
 )
 def test_apng_fdat(filename: str) -> None:
     with Image.open(filename) as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 255, 0, 255)
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
@@ -56,31 +59,37 @@ def test_apng_fdat(filename: str) -> None:
 
 def test_apng_dispose() -> None:
     with Image.open("Tests/images/apng/dispose_op_none.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 255, 0, 255)
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
     with Image.open("Tests/images/apng/dispose_op_background.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 0, 0, 0)
         assert im.getpixel((64, 32)) == (0, 0, 0, 0)
 
     with Image.open("Tests/images/apng/dispose_op_background_final.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 255, 0, 255)
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
     with Image.open("Tests/images/apng/dispose_op_previous.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 255, 0, 255)
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
     with Image.open("Tests/images/apng/dispose_op_previous_final.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 255, 0, 255)
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
     with Image.open("Tests/images/apng/dispose_op_previous_first.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 0, 0, 0)
         assert im.getpixel((64, 32)) == (0, 0, 0, 0)
@@ -88,21 +97,25 @@ def test_apng_dispose() -> None:
 
 def test_apng_dispose_region() -> None:
     with Image.open("Tests/images/apng/dispose_op_none_region.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 255, 0, 255)
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
     with Image.open("Tests/images/apng/dispose_op_background_before_region.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 0, 0, 0)
         assert im.getpixel((64, 32)) == (0, 0, 0, 0)
 
     with Image.open("Tests/images/apng/dispose_op_background_region.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 0, 255, 255)
         assert im.getpixel((64, 32)) == (0, 0, 0, 0)
 
     with Image.open("Tests/images/apng/dispose_op_previous_region.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 255, 0, 255)
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
@@ -129,6 +142,7 @@ def test_apng_dispose_op_previous_frame() -> None:
     #     ],
     # )
     with Image.open("Tests/images/apng/dispose_op_previous_frame.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (255, 0, 0, 255)
 
@@ -142,26 +156,31 @@ def test_apng_dispose_op_background_p_mode() -> None:
 
 def test_apng_blend() -> None:
     with Image.open("Tests/images/apng/blend_op_source_solid.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 255, 0, 255)
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
     with Image.open("Tests/images/apng/blend_op_source_transparent.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 0, 0, 0)
         assert im.getpixel((64, 32)) == (0, 0, 0, 0)
 
     with Image.open("Tests/images/apng/blend_op_source_near_transparent.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 255, 0, 2)
         assert im.getpixel((64, 32)) == (0, 255, 0, 2)
 
     with Image.open("Tests/images/apng/blend_op_over.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 255, 0, 255)
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
     with Image.open("Tests/images/apng/blend_op_over_near_transparent.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 255, 0, 97)
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
@@ -175,6 +194,7 @@ def test_apng_blend_transparency() -> None:
 
 def test_apng_chunk_order() -> None:
     with Image.open("Tests/images/apng/fctl_actl.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 255, 0, 255)
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
@@ -230,24 +250,28 @@ def test_apng_num_plays() -> None:
 
 def test_apng_mode() -> None:
     with Image.open("Tests/images/apng/mode_16bit.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert im.mode == "RGBA"
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (0, 0, 128, 191)
         assert im.getpixel((64, 32)) == (0, 0, 128, 191)
 
     with Image.open("Tests/images/apng/mode_grayscale.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert im.mode == "L"
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == 128
         assert im.getpixel((64, 32)) == 255
 
     with Image.open("Tests/images/apng/mode_grayscale_alpha.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert im.mode == "LA"
         im.seek(im.n_frames - 1)
         assert im.getpixel((0, 0)) == (128, 191)
         assert im.getpixel((64, 32)) == (128, 191)
 
     with Image.open("Tests/images/apng/mode_palette.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert im.mode == "P"
         im.seek(im.n_frames - 1)
         im = im.convert("RGB")
@@ -255,6 +279,7 @@ def test_apng_mode() -> None:
         assert im.getpixel((64, 32)) == (0, 255, 0)
 
     with Image.open("Tests/images/apng/mode_palette_alpha.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert im.mode == "P"
         im.seek(im.n_frames - 1)
         im = im.convert("RGBA")
@@ -262,6 +287,7 @@ def test_apng_mode() -> None:
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
     with Image.open("Tests/images/apng/mode_palette_1bit_alpha.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert im.mode == "P"
         im.seek(im.n_frames - 1)
         im = im.convert("RGBA")
@@ -271,25 +297,31 @@ def test_apng_mode() -> None:
 
 def test_apng_chunk_errors() -> None:
     with Image.open("Tests/images/apng/chunk_no_actl.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert not im.is_animated
 
     with pytest.warns(UserWarning):
         with Image.open("Tests/images/apng/chunk_multi_actl.png") as im:
             im.load()
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert not im.is_animated
 
     with Image.open("Tests/images/apng/chunk_actl_after_idat.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert not im.is_animated
 
     with Image.open("Tests/images/apng/chunk_no_fctl.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         with pytest.raises(SyntaxError):
             im.seek(im.n_frames - 1)
 
     with Image.open("Tests/images/apng/chunk_repeat_fctl.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         with pytest.raises(SyntaxError):
             im.seek(im.n_frames - 1)
 
     with Image.open("Tests/images/apng/chunk_no_fdat.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         with pytest.raises(SyntaxError):
             im.seek(im.n_frames - 1)
 
@@ -297,18 +329,21 @@ def test_apng_chunk_errors() -> None:
 def test_apng_syntax_errors() -> None:
     with pytest.warns(UserWarning):
         with Image.open("Tests/images/apng/syntax_num_frames_zero.png") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
             assert not im.is_animated
             with pytest.raises(OSError):
                 im.load()
 
     with pytest.warns(UserWarning):
         with Image.open("Tests/images/apng/syntax_num_frames_zero_default.png") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
             assert not im.is_animated
             im.load()
 
     # we can handle this case gracefully
     exception = None
     with Image.open("Tests/images/apng/syntax_num_frames_low.png") as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         try:
             im.seek(im.n_frames - 1)
         except Exception as e:
@@ -317,11 +352,13 @@ def test_apng_syntax_errors() -> None:
 
     with pytest.raises(OSError):
         with Image.open("Tests/images/apng/syntax_num_frames_high.png") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
             im.seek(im.n_frames - 1)
             im.load()
 
     with pytest.warns(UserWarning):
         with Image.open("Tests/images/apng/syntax_num_frames_invalid.png") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
             assert not im.is_animated
             im.load()
 
@@ -341,6 +378,7 @@ def test_apng_syntax_errors() -> None:
 def test_apng_sequence_errors(test_file: str) -> None:
     with pytest.raises(SyntaxError):
         with Image.open(f"Tests/images/apng/{test_file}") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
             im.seek(im.n_frames - 1)
             im.load()
 
@@ -351,6 +389,7 @@ def test_apng_save(tmp_path: Path) -> None:
         im.save(test_file, save_all=True)
 
     with Image.open(test_file) as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.load()
         assert not im.is_animated
         assert im.n_frames == 1
@@ -366,6 +405,7 @@ def test_apng_save(tmp_path: Path) -> None:
         )
 
     with Image.open(test_file) as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         im.load()
         assert im.is_animated
         assert im.n_frames == 2
@@ -405,6 +445,7 @@ def test_apng_save_split_fdat(tmp_path: Path) -> None:
             append_images=frames,
         )
     with Image.open(test_file) as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         exception = None
         try:
             im.seek(im.n_frames - 1)
@@ -452,6 +493,7 @@ def test_apng_save_duration_loop(tmp_path: Path) -> None:
         test_file, save_all=True, append_images=[frame, frame], duration=[500, 100, 150]
     )
     with Image.open(test_file) as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert im.n_frames == 1
         assert "duration" not in im.info
 
@@ -473,6 +515,7 @@ def test_apng_save_duration_loop(tmp_path: Path) -> None:
     frame.info["duration"] = 300
     frame.save(test_file, save_all=True, append_images=[frame, different_frame])
     with Image.open(test_file) as im:
+        assert isinstance(im, PngImagePlugin.PngImageFile)
         assert im.n_frames == 2
         assert im.info["duration"] == 600
 
diff --git a/Tests/test_file_dcx.py b/Tests/test_file_dcx.py
index 5deacd878ea..b041f8b7a40 100644
--- a/Tests/test_file_dcx.py
+++ b/Tests/test_file_dcx.py
@@ -69,12 +69,14 @@ def test_tell() -> None:
 
 def test_n_frames() -> None:
     with Image.open(TEST_FILE) as im:
+        assert isinstance(im, DcxImagePlugin.DcxImageFile)
         assert im.n_frames == 1
         assert not im.is_animated
 
 
 def test_eoferror() -> None:
     with Image.open(TEST_FILE) as im:
+        assert isinstance(im, DcxImagePlugin.DcxImageFile)
         n_frames = im.n_frames
 
         # Test seeking past the last frame
diff --git a/Tests/test_file_eps.py b/Tests/test_file_eps.py
index 672c04a4d4f..c745037f4f9 100644
--- a/Tests/test_file_eps.py
+++ b/Tests/test_file_eps.py
@@ -86,6 +86,8 @@
 def test_sanity(filename: str, size: tuple[int, int], scale: int) -> None:
     expected_size = tuple(s * scale for s in size)
     with Image.open(filename) as image:
+        assert isinstance(image, EpsImagePlugin.EpsImageFile)
+
         image.load(scale=scale)
         assert image.mode == "RGB"
         assert image.size == expected_size
@@ -223,6 +225,8 @@ def test_showpage() -> None:
 @pytest.mark.skipif(not HAS_GHOSTSCRIPT, reason="Ghostscript not available")
 def test_transparency() -> None:
     with Image.open("Tests/images/eps/reqd_showpage.eps") as plot_image:
+        assert isinstance(plot_image, EpsImagePlugin.EpsImageFile)
+
         plot_image.load(transparency=True)
         assert plot_image.mode == "RGBA"
 
@@ -304,6 +308,7 @@ def test_render_scale2() -> None:
 
     # Zero bounding box
     with Image.open(FILE1) as image1_scale2:
+        assert isinstance(image1_scale2, EpsImagePlugin.EpsImageFile)
         image1_scale2.load(scale=2)
         with Image.open(FILE1_COMPARE_SCALE2) as image1_scale2_compare:
             image1_scale2_compare = image1_scale2_compare.convert("RGB")
@@ -312,6 +317,7 @@ def test_render_scale2() -> None:
 
     # Non-zero bounding box
     with Image.open(FILE2) as image2_scale2:
+        assert isinstance(image2_scale2, EpsImagePlugin.EpsImageFile)
         image2_scale2.load(scale=2)
         with Image.open(FILE2_COMPARE_SCALE2) as image2_scale2_compare:
             image2_scale2_compare = image2_scale2_compare.convert("RGB")
diff --git a/Tests/test_file_fli.py b/Tests/test_file_fli.py
index 0a7740cc87d..302d3f94ccc 100644
--- a/Tests/test_file_fli.py
+++ b/Tests/test_file_fli.py
@@ -21,6 +21,8 @@
 
 def test_sanity() -> None:
     with Image.open(static_test_file) as im:
+        assert isinstance(im, FliImagePlugin.FliImageFile)
+
         im.load()
         assert im.mode == "P"
         assert im.size == (128, 128)
@@ -28,6 +30,8 @@ def test_sanity() -> None:
         assert not im.is_animated
 
     with Image.open(animated_test_file) as im:
+        assert isinstance(im, FliImagePlugin.FliImageFile)
+
         assert im.mode == "P"
         assert im.size == (320, 200)
         assert im.format == "FLI"
@@ -39,6 +43,8 @@ def test_prefix_chunk() -> None:
     ImageFile.LOAD_TRUNCATED_IMAGES = True
     try:
         with Image.open(animated_test_file_with_prefix_chunk) as im:
+            assert isinstance(im, FliImagePlugin.FliImageFile)
+
             assert im.mode == "P"
             assert im.size == (320, 200)
             assert im.format == "FLI"
@@ -114,16 +120,19 @@ def test_palette_chunk_second() -> None:
 
 def test_n_frames() -> None:
     with Image.open(static_test_file) as im:
+        assert isinstance(im, FliImagePlugin.FliImageFile)
         assert im.n_frames == 1
         assert not im.is_animated
 
     with Image.open(animated_test_file) as im:
+        assert isinstance(im, FliImagePlugin.FliImageFile)
         assert im.n_frames == 384
         assert im.is_animated
 
 
 def test_eoferror() -> None:
     with Image.open(animated_test_file) as im:
+        assert isinstance(im, FliImagePlugin.FliImageFile)
         n_frames = im.n_frames
 
         # Test seeking past the last frame
diff --git a/Tests/test_file_fpx.py b/Tests/test_file_fpx.py
index e32f30a0123..8d8064692e5 100644
--- a/Tests/test_file_fpx.py
+++ b/Tests/test_file_fpx.py
@@ -22,10 +22,11 @@ def test_sanity() -> None:
 
 def test_close() -> None:
     with Image.open("Tests/images/input_bw_one_band.fpx") as im:
-        pass
+        assert isinstance(im, FpxImagePlugin.FpxImageFile)
     assert im.ole.fp.closed
 
     im = Image.open("Tests/images/input_bw_one_band.fpx")
+    assert isinstance(im, FpxImagePlugin.FpxImageFile)
     im.close()
     assert im.ole.fp.closed
 
diff --git a/Tests/test_file_gif.py b/Tests/test_file_gif.py
index 5d46b157d55..93f2dbe0beb 100644
--- a/Tests/test_file_gif.py
+++ b/Tests/test_file_gif.py
@@ -282,6 +282,7 @@ def test_roundtrip_save_all(tmp_path: Path) -> None:
         im.save(out, save_all=True)
 
     with Image.open(out) as reread:
+        assert isinstance(reread, GifImagePlugin.GifImageFile)
         assert reread.n_frames == 5
 
 
@@ -443,10 +444,12 @@ def test_seek_rewind() -> None:
 def test_n_frames(path: str, n_frames: int) -> None:
     # Test is_animated before n_frames
     with Image.open(path) as im:
+        assert isinstance(im, GifImagePlugin.GifImageFile)
         assert im.is_animated == (n_frames != 1)
 
     # Test is_animated after n_frames
     with Image.open(path) as im:
+        assert isinstance(im, GifImagePlugin.GifImageFile)
         assert im.n_frames == n_frames
         assert im.is_animated == (n_frames != 1)
 
@@ -456,6 +459,7 @@ def test_no_change() -> None:
     with Image.open("Tests/images/dispose_bgnd.gif") as im:
         im.seek(1)
         expected = im.copy()
+        assert isinstance(im, GifImagePlugin.GifImageFile)
         assert im.n_frames == 5
         assert_image_equal(im, expected)
 
@@ -463,17 +467,20 @@ def test_no_change() -> None:
     with Image.open("Tests/images/dispose_bgnd.gif") as im:
         im.seek(3)
         expected = im.copy()
+        assert isinstance(im, GifImagePlugin.GifImageFile)
         assert im.is_animated
         assert_image_equal(im, expected)
 
     with Image.open("Tests/images/comment_after_only_frame.gif") as im:
         expected = Image.new("P", (1, 1))
+        assert isinstance(im, GifImagePlugin.GifImageFile)
         assert not im.is_animated
         assert_image_equal(im, expected)
 
 
 def test_eoferror() -> None:
     with Image.open(TEST_GIF) as im:
+        assert isinstance(im, GifImagePlugin.GifImageFile)
         n_frames = im.n_frames
 
         # Test seeking past the last frame
@@ -493,6 +500,7 @@ def test_first_frame_transparency() -> None:
 
 def test_dispose_none() -> None:
     with Image.open("Tests/images/dispose_none.gif") as img:
+        assert isinstance(img, GifImagePlugin.GifImageFile)
         try:
             while True:
                 img.seek(img.tell() + 1)
@@ -516,6 +524,7 @@ def test_dispose_none_load_end() -> None:
 
 def test_dispose_background() -> None:
     with Image.open("Tests/images/dispose_bgnd.gif") as img:
+        assert isinstance(img, GifImagePlugin.GifImageFile)
         try:
             while True:
                 img.seek(img.tell() + 1)
@@ -570,6 +579,7 @@ def test_transparent_dispose(
 
 def test_dispose_previous() -> None:
     with Image.open("Tests/images/dispose_prev.gif") as img:
+        assert isinstance(img, GifImagePlugin.GifImageFile)
         try:
             while True:
                 img.seek(img.tell() + 1)
@@ -607,6 +617,7 @@ def test_save_dispose(tmp_path: Path) -> None:
     for method in range(0, 4):
         im_list[0].save(out, save_all=True, append_images=im_list[1:], disposal=method)
         with Image.open(out) as img:
+            assert isinstance(img, GifImagePlugin.GifImageFile)
             for _ in range(2):
                 img.seek(img.tell() + 1)
                 assert img.disposal_method == method
@@ -620,6 +631,7 @@ def test_save_dispose(tmp_path: Path) -> None:
     )
 
     with Image.open(out) as img:
+        assert isinstance(img, GifImagePlugin.GifImageFile)
         for i in range(2):
             img.seek(img.tell() + 1)
             assert img.disposal_method == i + 1
@@ -742,6 +754,7 @@ def test_dispose2_background_frame(tmp_path: Path) -> None:
     im_list[0].save(out, save_all=True, append_images=im_list[1:], disposal=2)
 
     with Image.open(out) as im:
+        assert isinstance(im, GifImagePlugin.GifImageFile)
         assert im.n_frames == 3
 
 
@@ -908,6 +921,8 @@ def test_identical_frames(tmp_path: Path) -> None:
         out, save_all=True, append_images=im_list[1:], duration=duration_list
     )
     with Image.open(out) as reread:
+        assert isinstance(reread, GifImagePlugin.GifImageFile)
+
         # Assert that the first three frames were combined
         assert reread.n_frames == 2
 
@@ -1123,6 +1138,7 @@ def test_append_images(tmp_path: Path) -> None:
     im.copy().save(out, save_all=True, append_images=ims)
 
     with Image.open(out) as reread:
+        assert isinstance(reread, GifImagePlugin.GifImageFile)
         assert reread.n_frames == 3
 
     # Tests appending using a generator
@@ -1132,6 +1148,7 @@ def im_generator(ims: list[Image.Image]) -> Generator[Image.Image, None, None]:
     im.save(out, save_all=True, append_images=im_generator(ims))
 
     with Image.open(out) as reread:
+        assert isinstance(reread, GifImagePlugin.GifImageFile)
         assert reread.n_frames == 3
 
     # Tests appending single and multiple frame images
@@ -1140,6 +1157,7 @@ def im_generator(ims: list[Image.Image]) -> Generator[Image.Image, None, None]:
             im.save(out, save_all=True, append_images=[im2])
 
     with Image.open(out) as reread:
+        assert isinstance(reread, GifImagePlugin.GifImageFile)
         assert reread.n_frames == 10
 
 
@@ -1240,6 +1258,7 @@ def test_bbox(tmp_path: Path) -> None:
     im.save(out, save_all=True, append_images=ims)
 
     with Image.open(out) as reread:
+        assert isinstance(reread, GifImagePlugin.GifImageFile)
         assert reread.n_frames == 2
 
 
@@ -1252,6 +1271,7 @@ def test_bbox_alpha(tmp_path: Path) -> None:
     im.save(out, save_all=True, append_images=[im2])
 
     with Image.open(out) as reread:
+        assert isinstance(reread, GifImagePlugin.GifImageFile)
         assert reread.n_frames == 2
 
 
@@ -1313,6 +1333,7 @@ def test_palette_save_all_P(tmp_path: Path) -> None:
     with Image.open(out) as im:
         # Assert that the frames are correct, and each frame has the same palette
         assert_image_equal(im.convert("RGB"), frames[0].convert("RGB"))
+        assert isinstance(im, GifImagePlugin.GifImageFile)
         assert im.palette.palette == im.global_palette.palette
 
         im.seek(1)
@@ -1403,6 +1424,7 @@ def test_extents(
     GifImagePlugin.LOADING_STRATEGY = loading_strategy
     try:
         with Image.open("Tests/images/" + test_file) as im:
+            assert isinstance(im, GifImagePlugin.GifImageFile)
             assert im.size == (100, 100)
 
             # Check that n_frames does not change the size
@@ -1452,4 +1474,5 @@ def test_p_rgba(tmp_path: Path, params: dict[str, Any]) -> None:
     im1.save(out, save_all=True, append_images=[im2], **params)
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, GifImagePlugin.GifImageFile)
         assert reloaded.n_frames == 2
diff --git a/Tests/test_file_icns.py b/Tests/test_file_icns.py
index 141b88dfa01..57fa43a9370 100644
--- a/Tests/test_file_icns.py
+++ b/Tests/test_file_icns.py
@@ -65,6 +65,7 @@ def test_save_append_images(tmp_path: Path) -> None:
         assert_image_similar_tofile(im, temp_file, 1)
 
         with Image.open(temp_file) as reread:
+            assert isinstance(reread, IcnsImagePlugin.IcnsImageFile)
             reread.size = (16, 16)
             reread.load(2)
             assert_image_equal(reread, provided_im)
@@ -86,6 +87,7 @@ def test_sizes() -> None:
     # Check that we can load all of the sizes, and that the final pixel
     # dimensions are as expected
     with Image.open(TEST_FILE) as im:
+        assert isinstance(im, IcnsImagePlugin.IcnsImageFile)
         for w, h, r in im.info["sizes"]:
             wr = w * r
             hr = h * r
@@ -114,6 +116,7 @@ def test_older_icon() -> None:
             wr = w * r
             hr = h * r
             with Image.open("Tests/images/pillow2.icns") as im2:
+                assert isinstance(im2, IcnsImagePlugin.IcnsImageFile)
                 im2.size = (w, h)
                 im2.load(r)
                 assert im2.mode == "RGBA"
@@ -131,6 +134,7 @@ def test_jp2_icon() -> None:
             wr = w * r
             hr = h * r
             with Image.open("Tests/images/pillow3.icns") as im2:
+                assert isinstance(im2, IcnsImagePlugin.IcnsImageFile)
                 im2.size = (w, h)
                 im2.load(r)
                 assert im2.mode == "RGBA"
diff --git a/Tests/test_file_ico.py b/Tests/test_file_ico.py
index 37770498a0a..d63f41809ac 100644
--- a/Tests/test_file_ico.py
+++ b/Tests/test_file_ico.py
@@ -75,6 +75,7 @@ def test_save_to_bytes() -> None:
     # The other one
     output.seek(0)
     with Image.open(output) as reloaded:
+        assert isinstance(reloaded, IcoImagePlugin.IcoImageFile)
         reloaded.size = (32, 32)
 
         assert im.mode == reloaded.mode
@@ -92,6 +93,7 @@ def test_getpixel(tmp_path: Path) -> None:
     im.save(temp_file, "ico", sizes=[(32, 32), (64, 64)])
 
     with Image.open(temp_file) as reloaded:
+        assert isinstance(reloaded, IcoImagePlugin.IcoImageFile)
         reloaded.load()
         reloaded.size = (32, 32)
 
@@ -165,6 +167,7 @@ def test_save_to_bytes_bmp(mode: str) -> None:
     # The other one
     output.seek(0)
     with Image.open(output) as reloaded:
+        assert isinstance(reloaded, IcoImagePlugin.IcoImageFile)
         reloaded.size = (32, 32)
 
         assert "RGBA" == reloaded.mode
@@ -176,6 +179,7 @@ def test_save_to_bytes_bmp(mode: str) -> None:
 
 def test_incorrect_size() -> None:
     with Image.open(TEST_ICO_FILE) as im:
+        assert isinstance(im, IcoImagePlugin.IcoImageFile)
         with pytest.raises(ValueError):
             im.size = (1, 1)
 
@@ -217,6 +221,7 @@ def test_save_append_images(tmp_path: Path) -> None:
     im.save(outfile, sizes=[(32, 32), (128, 128)], append_images=[provided_im])
 
     with Image.open(outfile) as reread:
+        assert isinstance(reread, IcoImagePlugin.IcoImageFile)
         assert_image_equal(reread, hopper("RGBA"))
 
         reread.size = (32, 32)
diff --git a/Tests/test_file_im.py b/Tests/test_file_im.py
index 1d3fa485f3b..9830a9b4c82 100644
--- a/Tests/test_file_im.py
+++ b/Tests/test_file_im.py
@@ -68,12 +68,14 @@ def test_tell() -> None:
 
 def test_n_frames() -> None:
     with Image.open(TEST_IM) as im:
+        assert isinstance(im, ImImagePlugin.ImImageFile)
         assert im.n_frames == 1
         assert not im.is_animated
 
 
 def test_eoferror() -> None:
     with Image.open(TEST_IM) as im:
+        assert isinstance(im, ImImagePlugin.ImImageFile)
         n_frames = im.n_frames
 
         # Test seeking past the last frame
diff --git a/Tests/test_file_jpeg.py b/Tests/test_file_jpeg.py
index 347a162a58e..2d5ebcad10d 100644
--- a/Tests/test_file_jpeg.py
+++ b/Tests/test_file_jpeg.py
@@ -91,6 +91,7 @@ def test_zero(self, size: tuple[int, int], tmp_path: Path) -> None:
     def test_app(self) -> None:
         # Test APP/COM reader (@PIL135)
         with Image.open(TEST_FILE) as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
             assert im.applist[0] == ("APP0", b"JFIF\x00\x01\x01\x01\x00`\x00`\x00\x00")
             assert im.applist[1] == (
                 "COM",
@@ -309,6 +310,8 @@ def test_large_exif(self, tmp_path: Path) -> None:
 
     def test_exif_typeerror(self) -> None:
         with Image.open("Tests/images/exif_typeerror.jpg") as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
+
             # Should not raise a TypeError
             im._getexif()
 
@@ -324,6 +327,7 @@ def test_exif_gps(self, tmp_path: Path) -> None:
 
         # Reading
         with Image.open("Tests/images/exif_gps.jpg") as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
             exif = im._getexif()
             assert exif[gps_index] == expected_exif_gps
 
@@ -334,6 +338,7 @@ def test_exif_gps(self, tmp_path: Path) -> None:
         hopper().save(f, exif=exif)
 
         with Image.open(f) as reloaded:
+            assert isinstance(reloaded, JpegImagePlugin.JpegImageFile)
             exif = reloaded._getexif()
             assert exif[gps_index] == expected_exif_gps
 
@@ -363,6 +368,7 @@ def test_exif_equality(self) -> None:
         exifs = []
         for i in range(2):
             with Image.open("Tests/images/exif-200dpcm.jpg") as im:
+                assert isinstance(im, JpegImagePlugin.JpegImageFile)
                 exifs.append(im._getexif())
         assert exifs[0] == exifs[1]
 
@@ -396,6 +402,7 @@ def test_exif_rollback(self) -> None:
         }
 
         with Image.open("Tests/images/exif_gps.jpg") as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
             exif = im._getexif()
 
         for tag, value in expected_exif.items():
@@ -403,6 +410,8 @@ def test_exif_rollback(self) -> None:
 
     def test_exif_gps_typeerror(self) -> None:
         with Image.open("Tests/images/exif_gps_typeerror.jpg") as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
+
             # Should not raise a TypeError
             im._getexif()
 
@@ -478,6 +487,7 @@ def getsampling(
 
     def test_exif(self) -> None:
         with Image.open("Tests/images/pil_sample_rgb.jpg") as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
             info = im._getexif()
             assert info[305] == "Adobe Photoshop CS Macintosh"
 
@@ -490,6 +500,7 @@ def test_get_child_images(self) -> None:
 
     def test_mp(self) -> None:
         with Image.open("Tests/images/pil_sample_rgb.jpg") as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
             assert im._getmp() is None
 
     def test_quality_keep(self, tmp_path: Path) -> None:
@@ -547,12 +558,14 @@ def _n_qtables_helper(n: int, test_file: str) -> None:
             with Image.open(test_file) as im:
                 im.save(b, "JPEG", qtables=[[n] * 64] * n)
             with Image.open(b) as im:
+                assert isinstance(im, JpegImagePlugin.JpegImageFile)
                 assert len(im.quantization) == n
                 reloaded = self.roundtrip(im, qtables="keep")
                 assert im.quantization == reloaded.quantization
                 assert max(reloaded.quantization[0]) <= 255
 
         with Image.open("Tests/images/hopper.jpg") as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
             qtables = im.quantization
             reloaded = self.roundtrip(im, qtables=qtables, subsampling=0)
             assert im.quantization == reloaded.quantization
@@ -652,6 +665,7 @@ def _n_qtables_helper(n: int, test_file: str) -> None:
 
     def test_load_16bit_qtables(self) -> None:
         with Image.open("Tests/images/hopper_16bit_qtables.jpg") as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
             assert len(im.quantization) == 2
             assert len(im.quantization[0]) == 64
             assert max(im.quantization[0]) > 255
@@ -659,10 +673,12 @@ def test_load_16bit_qtables(self) -> None:
     def test_save_multiple_16bit_qtables(self) -> None:
         with Image.open("Tests/images/hopper_16bit_qtables.jpg") as im:
             im2 = self.roundtrip(im, qtables="keep")
+            assert isinstance(im2, JpegImagePlugin.JpegImageFile)
             assert im.quantization == im2.quantization
 
     def test_save_single_16bit_qtable(self) -> None:
         with Image.open("Tests/images/hopper_16bit_qtables.jpg") as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
             im2 = self.roundtrip(im, qtables={0: im.quantization[0]})
             assert len(im2.quantization) == 1
             assert im2.quantization[0] == im.quantization[0]
@@ -694,6 +710,7 @@ def test_restart_markers(self, blocks: int, rows: int, markers: int) -> None:
     @pytest.mark.skipif(not djpeg_available(), reason="djpeg not available")
     def test_load_djpeg(self) -> None:
         with Image.open(TEST_FILE) as img:
+            assert isinstance(img, JpegImagePlugin.JpegImageFile)
             img.load_djpeg()
             assert_image_similar_tofile(img, TEST_FILE, 5)
 
@@ -869,6 +886,8 @@ def test_ifd_offset_exif(self) -> None:
         # This image has been manually hexedited to have an IFD offset of 10,
         # in contrast to normal 8
         with Image.open("Tests/images/exif-ifd-offset.jpg") as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
+
             # Act / Assert
             assert im._getexif()[306] == "2017:03:13 23:03:09"
 
@@ -898,6 +917,7 @@ def test_photoshop(self) -> None:
 
     def test_photoshop_malformed_and_multiple(self) -> None:
         with Image.open("Tests/images/app13-multiple.jpg") as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
             assert "photoshop" in im.info
             assert 24 == len(im.info["photoshop"])
             apps_13_lengths = [len(v) for k, v in im.applist if k == "APP13"]
@@ -1069,6 +1089,7 @@ def test_repr_jpeg_error_returns_none(self) -> None:
 
     def test_deprecation(self) -> None:
         with Image.open(TEST_FILE) as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
             with pytest.warns(DeprecationWarning):
                 assert im.huffman_ac == {}
             with pytest.warns(DeprecationWarning):
diff --git a/Tests/test_file_jpeg2k.py b/Tests/test_file_jpeg2k.py
index fbf72ae0518..041ec5c8d73 100644
--- a/Tests/test_file_jpeg2k.py
+++ b/Tests/test_file_jpeg2k.py
@@ -230,12 +230,14 @@ def test_layers(card: ImageFile.ImageFile) -> None:
     out.seek(0)
 
     with Image.open(out) as im:
+        assert isinstance(im, Jpeg2KImagePlugin.Jpeg2KImageFile)
         im.layers = 1
         im.load()
         assert_image_similar(im, card, 13)
 
     out.seek(0)
     with Image.open(out) as im:
+        assert isinstance(im, Jpeg2KImagePlugin.Jpeg2KImageFile)
         im.layers = 3
         im.load()
         assert_image_similar(im, card, 0.4)
diff --git a/Tests/test_file_libtiff.py b/Tests/test_file_libtiff.py
index 9c49b1534ed..da84f6222cb 100644
--- a/Tests/test_file_libtiff.py
+++ b/Tests/test_file_libtiff.py
@@ -36,6 +36,7 @@ def _assert_noerr(self, tmp_path: Path, im: TiffImagePlugin.TiffImageFile) -> No
         im.load()
         im.getdata()
 
+        assert isinstance(im, TiffImagePlugin.TiffImageFile)
         try:
             assert im._compression == "group4"
         except AttributeError:
@@ -157,6 +158,7 @@ def test_write_metadata(self, legacy_api: bool, tmp_path: Path) -> None:
         """Test metadata writing through libtiff"""
         f = str(tmp_path / "temp.tiff")
         with Image.open("Tests/images/hopper_g4.tif") as img:
+            assert isinstance(img, TiffImagePlugin.TiffImageFile)
             img.save(f, tiffinfo=img.tag)
 
             if legacy_api:
@@ -174,6 +176,7 @@ def test_write_metadata(self, legacy_api: bool, tmp_path: Path) -> None:
         ]
 
         with Image.open(f) as loaded:
+            assert isinstance(loaded, TiffImagePlugin.TiffImageFile)
             if legacy_api:
                 reloaded = loaded.tag.named()
             else:
@@ -216,6 +219,7 @@ def test_additional_metadata(
         # Exclude ones that have special meaning
         # that we're already testing them
         with Image.open("Tests/images/hopper_g4.tif") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             for tag in im.tag_v2:
                 try:
                     del core_items[tag]
@@ -321,6 +325,7 @@ def check_tags(
             im.save(out, tiffinfo=tiffinfo)
 
             with Image.open(out) as reloaded:
+                assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
                 for tag, value in tiffinfo.items():
                     reloaded_value = reloaded.tag_v2[tag]
                     if (
@@ -353,12 +358,14 @@ def check_tags(
     def test_osubfiletype(self, tmp_path: Path) -> None:
         outfile = str(tmp_path / "temp.tif")
         with Image.open("Tests/images/g4_orientation_6.tif") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             im.tag_v2[OSUBFILETYPE] = 1
             im.save(outfile)
 
     def test_subifd(self, tmp_path: Path) -> None:
         outfile = str(tmp_path / "temp.tif")
         with Image.open("Tests/images/g4_orientation_6.tif") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             im.tag_v2[SUBIFD] = 10000
 
             # Should not segfault
@@ -373,6 +380,7 @@ def test_xmlpacket_tag(
         hopper().save(out, tiffinfo={700: b"xmlpacket tag"})
 
         with Image.open(out) as reloaded:
+            assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
             if 700 in reloaded.tag_v2:
                 assert reloaded.tag_v2[700] == b"xmlpacket tag"
 
@@ -434,12 +442,15 @@ def test_g4_string_info(self, tmp_path: Path) -> None:
         """Tests String data in info directory"""
         test_file = "Tests/images/hopper_g4_500.tif"
         with Image.open(test_file) as orig:
+            assert isinstance(orig, TiffImagePlugin.TiffImageFile)
+
             out = str(tmp_path / "temp.tif")
 
             orig.tag[269] = "temp.tif"
             orig.save(out)
 
         with Image.open(out) as reread:
+            assert isinstance(reread, TiffImagePlugin.TiffImageFile)
             assert "temp.tif" == reread.tag_v2[269]
             assert "temp.tif" == reread.tag[269][0]
 
@@ -545,6 +556,7 @@ def test_palette_save(
 
         with Image.open(out) as reloaded:
             # colormap/palette tag
+            assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
             assert len(reloaded.tag_v2[320]) == 768
 
     @pytest.mark.parametrize("compression", ("tiff_ccitt", "group3", "group4"))
@@ -576,6 +588,7 @@ def test_multipage(self, monkeypatch: pytest.MonkeyPatch) -> None:
         with Image.open("Tests/images/multipage.tiff") as im:
             # file is a multipage tiff,  10x10 green, 10x10 red, 20x20 blue
 
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             im.seek(0)
             assert im.size == (10, 10)
             assert im.convert("RGB").getpixel((0, 0)) == (0, 128, 0)
@@ -595,6 +608,7 @@ def test_multipage_nframes(self, monkeypatch: pytest.MonkeyPatch) -> None:
         # issue #862
         monkeypatch.setattr(TiffImagePlugin, "READ_LIBTIFF", True)
         with Image.open("Tests/images/multipage.tiff") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             frames = im.n_frames
             assert frames == 3
             for _ in range(frames):
@@ -614,6 +628,7 @@ def test_multipage_seek_backwards(self, monkeypatch: pytest.MonkeyPatch) -> None
     def test__next(self, monkeypatch: pytest.MonkeyPatch) -> None:
         monkeypatch.setattr(TiffImagePlugin, "READ_LIBTIFF", True)
         with Image.open("Tests/images/hopper.tif") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             assert not im.tag.next
             im.load()
             assert not im.tag.next
@@ -694,21 +709,25 @@ def test_save_ycbcr(self, tmp_path: Path) -> None:
         im.save(outfile, compression="jpeg")
 
         with Image.open(outfile) as reloaded:
+            assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
             assert reloaded.tag_v2[530] == (1, 1)
             assert reloaded.tag_v2[532] == (0, 255, 128, 255, 128, 255)
 
     def test_exif_ifd(self) -> None:
         out = io.BytesIO()
         with Image.open("Tests/images/tiff_adobe_deflate.tif") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             assert im.tag_v2[34665] == 125456
             im.save(out, "TIFF")
 
             with Image.open(out) as reloaded:
+                assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
                 assert 34665 not in reloaded.tag_v2
 
             im.save(out, "TIFF", tiffinfo={34665: 125456})
 
         with Image.open(out) as reloaded:
+            assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
             if Image.core.libtiff_support_custom_tags:
                 assert reloaded.tag_v2[34665] == 125456
 
@@ -790,6 +809,7 @@ def test_write_icc(
 
     def test_multipage_compression(self) -> None:
         with Image.open("Tests/images/compression.tif") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             im.seek(0)
             assert im._compression == "tiff_ccitt"
             assert im.size == (10, 10)
@@ -1083,6 +1103,7 @@ def test_orientation(self) -> None:
         with Image.open("Tests/images/g4_orientation_1.tif") as base_im:
             for i in range(2, 9):
                 with Image.open("Tests/images/g4_orientation_" + str(i) + ".tif") as im:
+                    assert isinstance(im, TiffImagePlugin.TiffImageFile)
                     assert 274 in im.tag_v2
 
                     im.load()
diff --git a/Tests/test_file_mic.py b/Tests/test_file_mic.py
index 9a6f13ea366..9aeb306e439 100644
--- a/Tests/test_file_mic.py
+++ b/Tests/test_file_mic.py
@@ -30,11 +30,13 @@ def test_sanity() -> None:
 
 def test_n_frames() -> None:
     with Image.open(TEST_FILE) as im:
+        assert isinstance(im, MicImagePlugin.MicImageFile)
         assert im.n_frames == 1
 
 
 def test_is_animated() -> None:
     with Image.open(TEST_FILE) as im:
+        assert isinstance(im, MicImagePlugin.MicImageFile)
         assert not im.is_animated
 
 
@@ -55,10 +57,11 @@ def test_seek() -> None:
 
 def test_close() -> None:
     with Image.open(TEST_FILE) as im:
-        pass
+        assert isinstance(im, MicImagePlugin.MicImageFile)
     assert im.ole.fp.closed
 
     im = Image.open(TEST_FILE)
+    assert isinstance(im, MicImagePlugin.MicImageFile)
     im.close()
     assert im.ole.fp.closed
 
diff --git a/Tests/test_file_mpo.py b/Tests/test_file_mpo.py
index 94958318557..e5ba5f54588 100644
--- a/Tests/test_file_mpo.py
+++ b/Tests/test_file_mpo.py
@@ -6,7 +6,7 @@
 
 import pytest
 
-from PIL import Image, ImageFile, MpoImagePlugin
+from PIL import Image, ImageFile, JpegImagePlugin, MpoImagePlugin
 
 from .helper import (
     assert_image_equal,
@@ -75,6 +75,7 @@ def test_context_manager() -> None:
 def test_app(test_file: str) -> None:
     # Test APP/COM reader (@PIL135)
     with Image.open(test_file) as im:
+        assert isinstance(im, MpoImagePlugin.MpoImageFile)
         assert im.applist[0][0] == "APP1"
         assert im.applist[1][0] == "APP2"
         assert (
@@ -114,6 +115,7 @@ def test_ignore_frame_size() -> None:
     # Ignore the different size of the second frame
     # since this is not a "Large Thumbnail" image
     with Image.open("Tests/images/ignore_frame_size.mpo") as im:
+        assert isinstance(im, MpoImagePlugin.MpoImageFile)
         assert im.size == (64, 64)
 
         im.seek(1)
@@ -149,6 +151,7 @@ def test_reload_exif_after_seek() -> None:
 @pytest.mark.parametrize("test_file", test_files)
 def test_mp(test_file: str) -> None:
     with Image.open(test_file) as im:
+        assert isinstance(im, MpoImagePlugin.MpoImageFile)
         mpinfo = im._getmp()
         assert mpinfo[45056] == b"0100"
         assert mpinfo[45057] == 2
@@ -158,6 +161,7 @@ def test_mp_offset() -> None:
     # This image has been manually hexedited to have an IFD offset of 10
     # in APP2 data, in contrast to normal 8
     with Image.open("Tests/images/sugarshack_ifd_offset.mpo") as im:
+        assert isinstance(im, MpoImagePlugin.MpoImageFile)
         mpinfo = im._getmp()
         assert mpinfo[45056] == b"0100"
         assert mpinfo[45057] == 2
@@ -174,6 +178,7 @@ def test_mp_no_data() -> None:
 @pytest.mark.parametrize("test_file", test_files)
 def test_mp_attribute(test_file: str) -> None:
     with Image.open(test_file) as im:
+        assert isinstance(im, MpoImagePlugin.MpoImageFile)
         mpinfo = im._getmp()
     for frame_number, mpentry in enumerate(mpinfo[0xB002]):
         mpattr = mpentry["Attribute"]
@@ -215,12 +220,14 @@ def test_seek(test_file: str) -> None:
 
 def test_n_frames() -> None:
     with Image.open("Tests/images/sugarshack.mpo") as im:
+        assert isinstance(im, MpoImagePlugin.MpoImageFile)
         assert im.n_frames == 2
         assert im.is_animated
 
 
 def test_eoferror() -> None:
     with Image.open("Tests/images/sugarshack.mpo") as im:
+        assert isinstance(im, MpoImagePlugin.MpoImageFile)
         n_frames = im.n_frames
 
         # Test seeking past the last frame
@@ -234,6 +241,8 @@ def test_eoferror() -> None:
 
 def test_adopt_jpeg() -> None:
     with Image.open("Tests/images/hopper.jpg") as im:
+        assert isinstance(im, JpegImagePlugin.JpegImageFile)
+
         with pytest.raises(ValueError):
             MpoImagePlugin.MpoImageFile.adopt(im)
 
diff --git a/Tests/test_file_png.py b/Tests/test_file_png.py
index ffafc3c582a..70574416fef 100644
--- a/Tests/test_file_png.py
+++ b/Tests/test_file_png.py
@@ -580,6 +580,7 @@ def test_getchunks(self) -> None:
 
     def test_read_private_chunks(self) -> None:
         with Image.open("Tests/images/exif.png") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
             assert im.private_chunks == [(b"orNT", b"\x01")]
 
     def test_roundtrip_private_chunk(self) -> None:
@@ -602,6 +603,7 @@ def test_roundtrip_private_chunk(self) -> None:
 
     def test_textual_chunks_after_idat(self) -> None:
         with Image.open("Tests/images/hopper.png") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
             assert "comment" in im.text
             for k, v in {
                 "date:create": "2014-09-04T09:37:08+03:00",
@@ -611,11 +613,14 @@ def test_textual_chunks_after_idat(self) -> None:
 
         # Raises a SyntaxError in load_end
         with Image.open("Tests/images/broken_data_stream.png") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
             with pytest.raises(OSError):
                 assert isinstance(im.text, dict)
 
         # Raises a UnicodeDecodeError in load_end
         with Image.open("Tests/images/truncated_image.png") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
+
             # The file is truncated
             with pytest.raises(OSError):
                 im.text()
@@ -625,6 +630,7 @@ def test_textual_chunks_after_idat(self) -> None:
 
         # Raises an EOFError in load_end
         with Image.open("Tests/images/hopper_idat_after_image_end.png") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
             assert im.text == {"TXT": "VALUE", "ZIP": "VALUE"}
 
     def test_unknown_compression_method(self) -> None:
@@ -669,6 +675,7 @@ def test_specify_bits(self, save_all: bool, tmp_path: Path) -> None:
         im.save(out, bits=4, save_all=save_all)
 
         with Image.open(out) as reloaded:
+            assert isinstance(reloaded, PngImagePlugin.PngImageFile)
             assert len(reloaded.png.im_palette[1]) == 48
 
     def test_plte_length(self, tmp_path: Path) -> None:
@@ -679,6 +686,7 @@ def test_plte_length(self, tmp_path: Path) -> None:
         im.save(str(tmp_path / "temp.png"))
 
         with Image.open(out) as reloaded:
+            assert isinstance(reloaded, PngImagePlugin.PngImageFile)
             assert len(reloaded.png.im_palette[1]) == 3
 
     def test_getxmp(self) -> None:
@@ -700,11 +708,13 @@ def test_getxmp(self) -> None:
     def test_exif(self) -> None:
         # With an EXIF chunk
         with Image.open("Tests/images/exif.png") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
             exif = im._getexif()
         assert exif[274] == 1
 
         # With an ImageMagick zTXt chunk
         with Image.open("Tests/images/exif_imagemagick.png") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
             exif = im._getexif()
             assert exif[274] == 1
 
@@ -715,6 +725,7 @@ def test_exif(self) -> None:
 
         # With a tEXt chunk
         with Image.open("Tests/images/exif_text.png") as im:
+            assert isinstance(im, PngImagePlugin.PngImageFile)
             exif = im._getexif()
         assert exif[274] == 1
 
@@ -730,6 +741,7 @@ def test_exif_save(self, tmp_path: Path) -> None:
             im.save(test_file)
 
         with Image.open(test_file) as reloaded:
+            assert isinstance(reloaded, PngImagePlugin.PngImageFile)
             assert reloaded._getexif() is None
 
         # Test passing in exif
@@ -737,6 +749,7 @@ def test_exif_save(self, tmp_path: Path) -> None:
             im.save(test_file, exif=im.getexif())
 
         with Image.open(test_file) as reloaded:
+            assert isinstance(reloaded, PngImagePlugin.PngImageFile)
             exif = reloaded._getexif()
         assert exif[274] == 1
 
@@ -749,6 +762,7 @@ def test_exif_from_jpg(self, tmp_path: Path) -> None:
             im.save(test_file, exif=im.getexif())
 
         with Image.open(test_file) as reloaded:
+            assert isinstance(reloaded, PngImagePlugin.PngImageFile)
             exif = reloaded._getexif()
         assert exif[305] == "Adobe Photoshop CS Macintosh"
 
diff --git a/Tests/test_file_psd.py b/Tests/test_file_psd.py
index 5f22001f324..90be6566433 100644
--- a/Tests/test_file_psd.py
+++ b/Tests/test_file_psd.py
@@ -59,17 +59,21 @@ def test_invalid_file() -> None:
 
 def test_n_frames() -> None:
     with Image.open("Tests/images/hopper_merged.psd") as im:
+        assert isinstance(im, PsdImagePlugin.PsdImageFile)
         assert im.n_frames == 1
         assert not im.is_animated
 
     for path in [test_file, "Tests/images/negative_layer_count.psd"]:
         with Image.open(path) as im:
+            assert isinstance(im, PsdImagePlugin.PsdImageFile)
             assert im.n_frames == 2
             assert im.is_animated
 
 
 def test_eoferror() -> None:
     with Image.open(test_file) as im:
+        assert isinstance(im, PsdImagePlugin.PsdImageFile)
+
         # PSD seek index starts at 1 rather than 0
         n_frames = im.n_frames + 1
 
@@ -119,11 +123,13 @@ def test_rgba() -> None:
 
 def test_negative_top_left_layer() -> None:
     with Image.open("Tests/images/negative_top_left_layer.psd") as im:
+        assert isinstance(im, PsdImagePlugin.PsdImageFile)
         assert im.layers[0][2] == (-50, -50, 50, 50)
 
 
 def test_layer_skip() -> None:
     with Image.open("Tests/images/five_channels.psd") as im:
+        assert isinstance(im, PsdImagePlugin.PsdImageFile)
         assert im.n_frames == 1
 
 
@@ -175,5 +181,6 @@ def test_crashes(test_file: str, raises: type[Exception]) -> None:
 def test_layer_crashes(test_file: str) -> None:
     with open(test_file, "rb") as f:
         with Image.open(f) as im:
+            assert isinstance(im, PsdImagePlugin.PsdImageFile)
             with pytest.raises(SyntaxError):
                 im.layers
diff --git a/Tests/test_file_spider.py b/Tests/test_file_spider.py
index 4cafda86536..89ee3b5856b 100644
--- a/Tests/test_file_spider.py
+++ b/Tests/test_file_spider.py
@@ -96,6 +96,7 @@ def test_tell() -> None:
 
 def test_n_frames() -> None:
     with Image.open(TEST_FILE) as im:
+        assert isinstance(im, SpiderImagePlugin.SpiderImageFile)
         assert im.n_frames == 1
         assert not im.is_animated
 
diff --git a/Tests/test_file_tiff.py b/Tests/test_file_tiff.py
index 6f51d46513e..bb9138f3900 100644
--- a/Tests/test_file_tiff.py
+++ b/Tests/test_file_tiff.py
@@ -9,7 +9,13 @@
 
 import pytest
 
-from PIL import Image, ImageFile, TiffImagePlugin, UnidentifiedImageError
+from PIL import (
+    Image,
+    ImageFile,
+    JpegImagePlugin,
+    TiffImagePlugin,
+    UnidentifiedImageError,
+)
 from PIL.TiffImagePlugin import RESOLUTION_UNIT, X_RESOLUTION, Y_RESOLUTION
 
 from .helper import (
@@ -113,6 +119,7 @@ def test_bigtiff(self, tmp_path: Path) -> None:
 
         with Image.open("Tests/images/hopper_bigtiff.tif") as im:
             outfile = str(tmp_path / "temp.tif")
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             im.save(outfile, save_all=True, append_images=[im], tiffinfo=im.tag_v2)
 
     def test_seek_too_large(self) -> None:
@@ -128,6 +135,8 @@ def test_set_legacy_api(self) -> None:
     def test_xyres_tiff(self) -> None:
         filename = "Tests/images/pil168.tif"
         with Image.open(filename) as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
+
             # legacy api
             assert isinstance(im.tag[X_RESOLUTION][0], tuple)
             assert isinstance(im.tag[Y_RESOLUTION][0], tuple)
@@ -141,6 +150,8 @@ def test_xyres_tiff(self) -> None:
     def test_xyres_fallback_tiff(self) -> None:
         filename = "Tests/images/compression.tif"
         with Image.open(filename) as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
+
             # v2 api
             assert isinstance(im.tag_v2[X_RESOLUTION], TiffImagePlugin.IFDRational)
             assert isinstance(im.tag_v2[Y_RESOLUTION], TiffImagePlugin.IFDRational)
@@ -155,6 +166,8 @@ def test_xyres_fallback_tiff(self) -> None:
     def test_int_resolution(self) -> None:
         filename = "Tests/images/pil168.tif"
         with Image.open(filename) as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
+
             # Try to read a file where X,Y_RESOLUTION are ints
             im.tag_v2[X_RESOLUTION] = 71
             im.tag_v2[Y_RESOLUTION] = 71
@@ -169,6 +182,7 @@ def test_load_float_dpi(self, resolution_unit: int | None, dpi: float) -> None:
         with Image.open(
             "Tests/images/hopper_float_dpi_" + str(resolution_unit) + ".tif"
         ) as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             assert im.tag_v2.get(RESOLUTION_UNIT) == resolution_unit
             assert im.info["dpi"] == (dpi, dpi)
 
@@ -186,6 +200,7 @@ def test_save_setting_missing_resolution(self) -> None:
         with Image.open("Tests/images/10ct_32bit_128.tiff") as im:
             im.save(b, format="tiff", resolution=123.45)
         with Image.open(b) as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             assert im.tag_v2[X_RESOLUTION] == 123.45
             assert im.tag_v2[Y_RESOLUTION] == 123.45
 
@@ -201,10 +216,12 @@ def test_invalid_file(self) -> None:
         TiffImagePlugin.PREFIXES.pop()
 
     def test_bad_exif(self) -> None:
-        with Image.open("Tests/images/hopper_bad_exif.jpg") as i:
+        with Image.open("Tests/images/hopper_bad_exif.jpg") as im:
+            assert isinstance(im, JpegImagePlugin.JpegImageFile)
+
             # Should not raise struct.error.
             with pytest.warns(UserWarning):
-                i._getexif()
+                im._getexif()
 
     def test_save_rgba(self, tmp_path: Path) -> None:
         im = hopper("RGBA")
@@ -295,11 +312,13 @@ def test_unknown_pixel_mode(self) -> None:
     )
     def test_n_frames(self, path: str, n_frames: int) -> None:
         with Image.open(path) as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             assert im.n_frames == n_frames
             assert im.is_animated == (n_frames != 1)
 
     def test_eoferror(self) -> None:
         with Image.open("Tests/images/multipage-lastframe.tif") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             n_frames = im.n_frames
 
             # Test seeking past the last frame
@@ -343,19 +362,24 @@ def test_multipage_last_frame(self) -> None:
     def test_frame_order(self) -> None:
         # A frame can't progress to itself after reading
         with Image.open("Tests/images/multipage_single_frame_loop.tiff") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             assert im.n_frames == 1
 
         # A frame can't progress to a frame that has already been read
         with Image.open("Tests/images/multipage_multiple_frame_loop.tiff") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             assert im.n_frames == 2
 
         # Frames don't have to be in sequence
         with Image.open("Tests/images/multipage_out_of_order.tiff") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             assert im.n_frames == 3
 
     def test___str__(self) -> None:
         filename = "Tests/images/pil136.tiff"
         with Image.open(filename) as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
+
             # Act
             ret = str(im.ifd)
 
@@ -366,6 +390,8 @@ def test_dict(self) -> None:
         # Arrange
         filename = "Tests/images/pil136.tiff"
         with Image.open(filename) as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
+
             # v2 interface
             v2_tags = {
                 256: 55,
@@ -405,6 +431,7 @@ def test_dict(self) -> None:
     def test__delitem__(self) -> None:
         filename = "Tests/images/pil136.tiff"
         with Image.open(filename) as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             len_before = len(dict(im.ifd))
             del im.ifd[256]
             len_after = len(dict(im.ifd))
@@ -437,6 +464,7 @@ def test_load_double(self) -> None:
 
     def test_ifd_tag_type(self) -> None:
         with Image.open("Tests/images/ifd_tag_type.tiff") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             assert 0x8825 in im.tag_v2
 
     def test_exif(self, tmp_path: Path) -> None:
@@ -525,6 +553,7 @@ def test_photometric(self, mode: str, tmp_path: Path) -> None:
         im = hopper(mode)
         im.save(filename, tiffinfo={262: 0})
         with Image.open(filename) as reloaded:
+            assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
             assert reloaded.tag_v2[262] == 0
             assert_image_equal(im, reloaded)
 
@@ -603,6 +632,8 @@ def test_with_underscores(self, tmp_path: Path) -> None:
         filename = str(tmp_path / "temp.tif")
         hopper("RGB").save(filename, "TIFF", **kwargs)
         with Image.open(filename) as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
+
             # legacy interface
             assert im.tag[X_RESOLUTION][0][0] == 72
             assert im.tag[Y_RESOLUTION][0][0] == 36
@@ -677,6 +708,7 @@ def test_tiled_planar_raw(self) -> None:
     def test_planar_configuration_save(self, tmp_path: Path) -> None:
         infile = "Tests/images/tiff_tiled_planar_raw.tif"
         with Image.open(infile) as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             assert im._planar_configuration == 2
 
             outfile = str(tmp_path / "temp.tif")
@@ -709,6 +741,7 @@ def test_tiff_save_all(self) -> None:
 
         mp.seek(0, os.SEEK_SET)
         with Image.open(mp) as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             assert im.n_frames == 3
 
         # Test appending images
@@ -719,6 +752,7 @@ def test_tiff_save_all(self) -> None:
 
         mp.seek(0, os.SEEK_SET)
         with Image.open(mp) as reread:
+            assert isinstance(reread, TiffImagePlugin.TiffImageFile)
             assert reread.n_frames == 3
 
         # Test appending using a generator
@@ -730,6 +764,7 @@ def im_generator(ims: list[Image.Image]) -> Generator[Image.Image, None, None]:
 
         mp.seek(0, os.SEEK_SET)
         with Image.open(mp) as reread:
+            assert isinstance(reread, TiffImagePlugin.TiffImageFile)
             assert reread.n_frames == 3
 
     def test_fixoffsets(self) -> None:
@@ -807,6 +842,7 @@ def test_getxmp(self) -> None:
 
     def test_get_photoshop_blocks(self) -> None:
         with Image.open("Tests/images/lab.tif") as im:
+            assert isinstance(im, TiffImagePlugin.TiffImageFile)
             assert list(im.get_photoshop_blocks().keys()) == [
                 1061,
                 1002,
diff --git a/Tests/test_file_tiff_metadata.py b/Tests/test_file_tiff_metadata.py
index 36aabf4f82c..1e6cd20c3f0 100644
--- a/Tests/test_file_tiff_metadata.py
+++ b/Tests/test_file_tiff_metadata.py
@@ -61,6 +61,7 @@ def test_rt_metadata(tmp_path: Path) -> None:
     img.save(f, tiffinfo=info)
 
     with Image.open(f) as loaded:
+        assert isinstance(loaded, TiffImagePlugin.TiffImageFile)
         assert loaded.tag[ImageJMetaDataByteCounts] == (len(bin_data),)
         assert loaded.tag_v2[ImageJMetaDataByteCounts] == (len(bin_data),)
 
@@ -80,12 +81,14 @@ def test_rt_metadata(tmp_path: Path) -> None:
     info[ImageJMetaDataByteCounts] = (8, len(bin_data) - 8)
     img.save(f, tiffinfo=info)
     with Image.open(f) as loaded:
+        assert isinstance(loaded, TiffImagePlugin.TiffImageFile)
         assert loaded.tag[ImageJMetaDataByteCounts] == (8, len(bin_data) - 8)
         assert loaded.tag_v2[ImageJMetaDataByteCounts] == (8, len(bin_data) - 8)
 
 
 def test_read_metadata() -> None:
     with Image.open("Tests/images/hopper_g4.tif") as img:
+        assert isinstance(img, TiffImagePlugin.TiffImageFile)
         assert {
             "YResolution": IFDRational(4294967295, 113653537),
             "PlanarConfiguration": 1,
@@ -128,6 +131,7 @@ def test_read_metadata() -> None:
 def test_write_metadata(tmp_path: Path) -> None:
     """Test metadata writing through the python code"""
     with Image.open("Tests/images/hopper.tif") as img:
+        assert isinstance(img, TiffImagePlugin.TiffImageFile)
         f = str(tmp_path / "temp.tiff")
         del img.tag[278]
         img.save(f, tiffinfo=img.tag)
@@ -135,6 +139,7 @@ def test_write_metadata(tmp_path: Path) -> None:
         original = img.tag_v2.named()
 
     with Image.open(f) as loaded:
+        assert isinstance(loaded, TiffImagePlugin.TiffImageFile)
         reloaded = loaded.tag_v2.named()
 
     ignored = ["StripByteCounts", "RowsPerStrip", "PageNumber", "StripOffsets"]
@@ -165,6 +170,7 @@ def test_write_metadata(tmp_path: Path) -> None:
 def test_change_stripbytecounts_tag_type(tmp_path: Path) -> None:
     out = str(tmp_path / "temp.tiff")
     with Image.open("Tests/images/hopper.tif") as im:
+        assert isinstance(im, TiffImagePlugin.TiffImageFile)
         info = im.tag_v2
         del info[278]
 
@@ -178,6 +184,7 @@ def test_change_stripbytecounts_tag_type(tmp_path: Path) -> None:
         im.save(out, tiffinfo=info)
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert reloaded.tag_v2.tagtype[TiffImagePlugin.STRIPBYTECOUNTS] == TiffTags.LONG
 
 
@@ -231,6 +238,7 @@ def test_writing_other_types_to_ascii(
     im.save(out, tiffinfo=info)
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert reloaded.tag_v2[271] == expected
 
 
@@ -248,6 +256,7 @@ def test_writing_other_types_to_bytes(value: int | IFDRational, tmp_path: Path)
     im.save(out, tiffinfo=info)
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert reloaded.tag_v2[700] == b"\x01"
 
 
@@ -267,6 +276,7 @@ def test_writing_other_types_to_undefined(
     im.save(out, tiffinfo=info)
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert reloaded.tag_v2[33723] == b"1"
 
 
@@ -311,6 +321,7 @@ def test_iccprofile_binary() -> None:
     # but probably won't be able to save it.
 
     with Image.open("Tests/images/hopper.iccprofile_binary.tif") as im:
+        assert isinstance(im, TiffImagePlugin.TiffImageFile)
         assert im.tag_v2.tagtype[34675] == 1
         assert im.info["icc_profile"]
 
@@ -336,6 +347,7 @@ def test_exif_div_zero(tmp_path: Path) -> None:
     im.save(out, tiffinfo=info, compression="raw")
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert 0 == reloaded.tag_v2[41988].numerator
         assert 0 == reloaded.tag_v2[41988].denominator
 
@@ -355,6 +367,7 @@ def test_ifd_unsigned_rational(tmp_path: Path) -> None:
     im.save(out, tiffinfo=info, compression="raw")
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert max_long == reloaded.tag_v2[41493].numerator
         assert 1 == reloaded.tag_v2[41493].denominator
 
@@ -367,6 +380,7 @@ def test_ifd_unsigned_rational(tmp_path: Path) -> None:
     im.save(out, tiffinfo=info, compression="raw")
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert max_long == reloaded.tag_v2[41493].numerator
         assert 1 == reloaded.tag_v2[41493].denominator
 
@@ -385,6 +399,7 @@ def test_ifd_signed_rational(tmp_path: Path) -> None:
     im.save(out, tiffinfo=info, compression="raw")
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert numerator == reloaded.tag_v2[37380].numerator
         assert denominator == reloaded.tag_v2[37380].denominator
 
@@ -397,6 +412,7 @@ def test_ifd_signed_rational(tmp_path: Path) -> None:
     im.save(out, tiffinfo=info, compression="raw")
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert numerator == reloaded.tag_v2[37380].numerator
         assert denominator == reloaded.tag_v2[37380].denominator
 
@@ -410,6 +426,7 @@ def test_ifd_signed_rational(tmp_path: Path) -> None:
     im.save(out, tiffinfo=info, compression="raw")
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert 2**31 - 1 == reloaded.tag_v2[37380].numerator
         assert -1 == reloaded.tag_v2[37380].denominator
 
@@ -424,6 +441,7 @@ def test_ifd_signed_long(tmp_path: Path) -> None:
     im.save(out, tiffinfo=info, compression="raw")
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert reloaded.tag_v2[37000] == -60000
 
 
@@ -444,11 +462,13 @@ def test_empty_values() -> None:
 
 def test_photoshop_info(tmp_path: Path) -> None:
     with Image.open("Tests/images/issue_2278.tif") as im:
+        assert isinstance(im, TiffImagePlugin.TiffImageFile)
         assert len(im.tag_v2[34377]) == 70
         assert isinstance(im.tag_v2[34377], bytes)
         out = str(tmp_path / "temp.tiff")
         im.save(out)
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert len(reloaded.tag_v2[34377]) == 70
         assert isinstance(reloaded.tag_v2[34377], bytes)
 
diff --git a/Tests/test_file_webp_animated.py b/Tests/test_file_webp_animated.py
index 967a0aae847..9035d905792 100644
--- a/Tests/test_file_webp_animated.py
+++ b/Tests/test_file_webp_animated.py
@@ -6,7 +6,7 @@
 import pytest
 from packaging.version import parse as parse_version
 
-from PIL import Image, features
+from PIL import GifImagePlugin, Image, WebPImagePlugin, features
 
 from .helper import (
     assert_image_equal,
@@ -22,10 +22,12 @@ def test_n_frames() -> None:
     """Ensure that WebP format sets n_frames and is_animated attributes correctly."""
 
     with Image.open("Tests/images/hopper.webp") as im:
+        assert isinstance(im, WebPImagePlugin.WebPImageFile)
         assert im.n_frames == 1
         assert not im.is_animated
 
     with Image.open("Tests/images/iss634.webp") as im:
+        assert isinstance(im, WebPImagePlugin.WebPImageFile)
         assert im.n_frames == 42
         assert im.is_animated
 
@@ -37,11 +39,13 @@ def test_write_animation_L(tmp_path: Path) -> None:
     """
 
     with Image.open("Tests/images/iss634.gif") as orig:
+        assert isinstance(orig, GifImagePlugin.GifImageFile)
         assert orig.n_frames > 1
 
         temp_file = str(tmp_path / "temp.webp")
         orig.save(temp_file, save_all=True)
         with Image.open(temp_file) as im:
+            assert isinstance(im, WebPImagePlugin.WebPImageFile)
             assert im.n_frames == orig.n_frames
 
             # Compare first and last frames to the original animated GIF
@@ -69,6 +73,7 @@ def test_write_animation_RGB(tmp_path: Path) -> None:
 
     def check(temp_file: str) -> None:
         with Image.open(temp_file) as im:
+            assert isinstance(im, WebPImagePlugin.WebPImageFile)
             assert im.n_frames == 2
 
             # Compare first frame to original
@@ -127,6 +132,7 @@ def test_timestamp_and_duration(tmp_path: Path) -> None:
             )
 
     with Image.open(temp_file) as im:
+        assert isinstance(im, WebPImagePlugin.WebPImageFile)
         assert im.n_frames == 5
         assert im.is_animated
 
@@ -170,6 +176,7 @@ def test_seeking(tmp_path: Path) -> None:
             )
 
     with Image.open(temp_file) as im:
+        assert isinstance(im, WebPImagePlugin.WebPImageFile)
         assert im.n_frames == 5
         assert im.is_animated
 
diff --git a/Tests/test_file_webp_metadata.py b/Tests/test_file_webp_metadata.py
index d9a834c7500..159bd96ae99 100644
--- a/Tests/test_file_webp_metadata.py
+++ b/Tests/test_file_webp_metadata.py
@@ -6,7 +6,7 @@
 
 import pytest
 
-from PIL import Image
+from PIL import Image, WebPImagePlugin
 
 from .helper import mark_if_feature_version, skip_unless_feature
 
@@ -22,6 +22,7 @@
 def test_read_exif_metadata() -> None:
     file_path = "Tests/images/flower.webp"
     with Image.open(file_path) as image:
+        assert isinstance(image, WebPImagePlugin.WebPImageFile)
         assert image.format == "WEBP"
         exif_data = image.info.get("exif", None)
         assert exif_data
@@ -110,6 +111,7 @@ def test_read_no_exif() -> None:
 
     test_buffer.seek(0)
     with Image.open(test_buffer) as webp_image:
+        assert isinstance(webp_image, WebPImagePlugin.WebPImageFile)
         assert not webp_image._getexif()
 
 
diff --git a/Tests/test_file_wmf.py b/Tests/test_file_wmf.py
index 2f1f8cdbc85..3dc8e06bf10 100644
--- a/Tests/test_file_wmf.py
+++ b/Tests/test_file_wmf.py
@@ -78,6 +78,7 @@ def test_load_float_dpi() -> None:
 
 def test_load_set_dpi() -> None:
     with Image.open("Tests/images/drawing.wmf") as im:
+        assert isinstance(im, WmfImagePlugin.WmfStubImageFile)
         assert im.size == (82, 82)
 
         if hasattr(Image.core, "drawwmf"):
diff --git a/Tests/test_file_xpm.py b/Tests/test_file_xpm.py
index 26afe93f450..73c62a44d53 100644
--- a/Tests/test_file_xpm.py
+++ b/Tests/test_file_xpm.py
@@ -30,6 +30,7 @@ def test_invalid_file() -> None:
 def test_load_read() -> None:
     # Arrange
     with Image.open(TEST_FILE) as im:
+        assert isinstance(im, XpmImagePlugin.XpmImageFile)
         dummy_bytes = 1
 
         # Act
diff --git a/Tests/test_imagesequence.py b/Tests/test_imagesequence.py
index 9b37435eb61..7f55f01f69d 100644
--- a/Tests/test_imagesequence.py
+++ b/Tests/test_imagesequence.py
@@ -4,7 +4,7 @@
 
 import pytest
 
-from PIL import Image, ImageSequence, TiffImagePlugin
+from PIL import Image, ImageSequence, PsdImagePlugin, TiffImagePlugin
 
 from .helper import assert_image_equal, hopper, skip_unless_feature
 
@@ -31,6 +31,7 @@ def test_sanity(tmp_path: Path) -> None:
 
 def test_iterator() -> None:
     with Image.open("Tests/images/multipage.tiff") as im:
+        assert isinstance(im, TiffImagePlugin.TiffImageFile)
         i = ImageSequence.Iterator(im)
         for index in range(0, im.n_frames):
             assert i[index] == next(i)
@@ -42,6 +43,7 @@ def test_iterator() -> None:
 
 def test_iterator_min_frame() -> None:
     with Image.open("Tests/images/hopper.psd") as im:
+        assert isinstance(im, PsdImagePlugin.PsdImageFile)
         i = ImageSequence.Iterator(im)
         for index in range(1, im.n_frames):
             assert i[index] == next(i)
diff --git a/Tests/test_shell_injection.py b/Tests/test_shell_injection.py
index dd4fc46c374..eb5d3388db9 100644
--- a/Tests/test_shell_injection.py
+++ b/Tests/test_shell_injection.py
@@ -39,6 +39,7 @@ def test_load_djpeg_filename(self, tmp_path: Path) -> None:
             shutil.copy(TEST_JPG, src_file)
 
             with Image.open(src_file) as im:
+                assert isinstance(im, JpegImagePlugin.JpegImageFile)
                 im.load_djpeg()
 
     @pytest.mark.skipif(not cjpeg_available(), reason="cjpeg not available")
diff --git a/Tests/test_tiff_ifdrational.py b/Tests/test_tiff_ifdrational.py
index 13f1f9c80fe..6df1e455708 100644
--- a/Tests/test_tiff_ifdrational.py
+++ b/Tests/test_tiff_ifdrational.py
@@ -72,4 +72,5 @@ def test_ifd_rational_save(
     im.save(out, dpi=(res, res), compression="raw")
 
     with Image.open(out) as reloaded:
+        assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
         assert float(IFDRational(301, 1)) == float(reloaded.tag_v2[282])