[BUGFIX] Followup: Ajax handler TYPO3_tcefile::process is broken

Upload action was taken care of, but the ajax handler can be just
for all commands that ExtendedFileUtility->processData can handle.

This change checks the result set and flattens
data only when needed.

Resolves: #56084
Releases: 6.2, 6.1
Change-Id: Ic1a0bd9084b9eb206b9b53960890d22d2a9c56f5
Reviewed-on: https://review.typo3.org/27739
Reviewed-by: Alexander Schnitzler
Tested-by: Alexander Schnitzler
Reviewed-by: Markus Klein
Tested-by: Markus Klein