[BUGFIX] FILES.folder does not work

cObject IMAGE was executed with empty values
because given key contains filenames instead of
numbers

Resolves: #45724
Releases: 6.2, 6.1, 6.0
Change-Id: I5582b63080b7e6124c3780a8430efb410876a662
Reviewed-on: https://review.typo3.org/25411
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind