[BUGFIX] Unable to edit file metadata

This patch adjusts the handling of the file field after FormEngine
processing.

Resolves: #70553
Releases: master
Change-Id: Ie201c7df5ac74a95c3549430bfdea6835b4f4656
Reviewed-on: http://review.typo3.org/43945


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>