Fixed #14760 - Admin inlines with file/image field fails to save_as

https://github.com/django/django/pull/697 reworked for master.

Refactored BaseInlineFormSet and ModelAdmin to copy initial
values (file name) for fields of FileField or it's subclasses.

'physical' file do not get duplicated.
Would need to update documentation for the change in the signature (docs/ref/contrib/admin/index.txt). I'm not convinced "original" is a good name though.