diff --git a/src/PIL/Image.py b/src/PIL/Image.py
index e2d460eeb3b..2a6b4646bbd 100644
--- a/src/PIL/Image.py
+++ b/src/PIL/Image.py
@@ -3409,8 +3409,12 @@ def register_open(id, factory, accept=None):
 
 def register_mime(id, mimetype):
     """
-    Registers an image MIME type.  This function should not be used
-    in application code.
+    Registers an image MIME type by populating ``Image.MIME``. This function
+    should not be used in application code.
+
+    ``Image.MIME`` provides a mapping from image format identifiers to mime
+    formats, but :py:meth:`~PIL.ImageFile.ImageFile.get_format_mimetype` can
+    provide a different result for specific images.
 
     :param id: An image format identifier.
     :param mimetype: The image MIME type for this format.