🐛 [#4795] Validate .msg file type only on backend

The sdk cannot determine which content type belongs to a .msg file. This is because (at least) Linux and MacOS don't know this file type.

To make sure these files can be uploaded, the type property on the FileSerializer is now optional. For .smg files a new rule has been added to the MimeTypeValidator

src/openforms/formio/api/validators.py

@@ -111,6 +111,11 @@ def __call__(self, value: UploadedFile) -> None:
             "image/heif",
         ):
             return
+        # 4795
+        # The sdk cannot determine the file type of .msg files, which result into
+        # content_type "". So we have to validate these for ourselves
+        elif mime_type == "application/vnd.ms-outlook" and ext == "msg":
+            return
 
         # gh #4658
         # Windows use application/x-zip-compressed as a mimetype for .zip files, which

src/openforms/formio/components/vanilla.py

@@ -338,14 +338,7 @@ class FileSerializer(serializers.Serializer):
     originalName = serializers.CharField(trim_whitespace=False)
     size = serializers.IntegerField(min_value=0)
     storage = serializers.ChoiceField(choices=["url"])
-    type = serializers.CharField(
-        error_messages={
-            "blank": _(
-                "Could not determine the file type. Please make sure the file name "
-                "has an extension."
-            ),
-        }
-    )
+    type = serializers.CharField(required=False, allow_blank=True)
     url = serializers.URLField()
     data = FileDataSerializer()  # type: ignore