[BUGFIX] Fix bug in linkhandler when file is selected

Selecting a file an the RTE linkhandler works but reopening
the linkhandler ends with a fatal error as a file object
is passed instead of string.

Change-Id: I56972ef8d25e9add0c51e1b72fd7034c1140bc01
Resolves: #78782
Releases: master
Reviewed-on: https://review.typo3.org/50767


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Markus Sommer <markussom@posteo.de>
Tested-by: Markus Sommer <markussom@posteo.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>