[BUGFIX] Handle file_references properly in t3lib_befunc::thumbCode

Group fields with internal_type "file_reference" reference the files
directly. The filepaths don't need any additional uploadFolder prefixes.

Due to the FAL changes in that method, this patch is not relevant for 6.0.

Change-Id: I7428fb9561180dd51cc1f45a9f6c012626b72fa8
Resolves: #31295
Releases: 4.5
Reviewed-on: https://review.typo3.org/21934
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind