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.
Also, I see this pull request should need to be rebased for master.