File(allow_none=True) is not possible

Hi team,

Is there interest in making `File(allow_none=True)`?
This would allow to send `None` to file and it being validated as valid.
The use case is to delete the file when editing a resource (set it to None to delete it).
I can make pull request.

Start by locating the File field implementation and its validation tests, then trace how allow_none is handled. Done means File(allow_none=True) accepts None for the stated edit/delete use case, with regression coverage for that behavior.

Written by the indexing model from the issue text.