[BUGFIX] Fix refindex for FlexForm fields type group file_reference

According to the fixes for the normal TCA fields of type "group"
internal_type "file_reference" in #49538 and #56353, this needs also
to be fixed for the FlexForm fields: handle the internal_type
file_reference as an db reference to sys_file.

Resolves: #56991
Releases: 6.2, 6.1, 6.0
Change-Id: Ie66c86c1bf3f0386d23259d0aee6706564beace3
Reviewed-on: https://review.typo3.org/28797
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs