[BUGFIX] Add autocomplete for t3editor FILES

Add autocomplete values for t3editor
because there was no implementation for new cObj FILES

Resolves: #53129
Releases: master, 6.2
Change-Id: Id4c5d7e51850546cb07ae289c96107834b8e7921
Reviewed-on: http://review.typo3.org/35394
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>