Fix issue caused by Django 5.1 added FileField name validation

[dbogar89]

Problem

Django 5.1 introduced a breaking change in FileField.pre_save() that raises a FieldError when trying to save a file without a name attribute:
https://docs.djangoproject.com/en/5.2/releases/5.1/#miscellaneous

This caused S3FileField to fail with the error "str has no attribute .name" because the existing save_form_data method converts S3PlaceholderFile objects to strings before passing them to the parent class.

Solution

Modified save_form_data to handle S3PlaceholderFile objects by directly setting the field attribute using setattr, bypassing Django's FileField validation while maintaining the same behaviour of storing the filename as a string in the database.

[dbogar89]

Did you get the chance to look at this @brianhelba?