Cannot merge models with subclassed ImageField or FileField: "file not sent" error

[int-ua]

If a model that I want to merge have a field that is not an original ImageFIeld, but was subclassed from it, for example:

class ImageField2(ImageField):
    pass

then default MergeForm stops working if resulting instance contains a link in this field. Is it a bug? Can I somehow subclass MergeForm to state which field needs to be just copied instead of trying to upload an image?

It's an old project with django-thumbs, in case it can help.

[int-ua]

Note to self: it's not limited to Grappelli, fails without it too.

[int-ua]

The error occurs on this line, FileFields also affected https://github.com/django/django/blob/d3d12fc11da56e4ea8af37a22a9a0aa6579ab2d5/django/forms/forms.py#L405 in django.forms.forms.BaseForm._clean_fields()

[int-ua]

I can imagine two possible workarounds:

• changing adminactions.merge.merge.raw_widget so that it won't return TextWidget if isinstance(field, models.FileField). This breaks value selections before merging.
• writing custom to_python method for the subclassed field that will reconstruct UploadedFile from str