Modelform cant inhertit from forms. To solve this issue, split thing you wanto to inherit into filed definition and functionality definition. For modelform use the base_fields.update method as mentioned in the code.
I needed to overwrite files on update (not create new ones) but also a validation which would prevent to upload 2 files with the same name. The CustomCheckFiled triggers a validation passing the filename and model instance. If the validation returns false, the validation error_message will be displayed in admin. The OverwriteStorage is needed because the default storage alters the name if such name already exists. Enjoy.
evilclay has posted 2 snippets.