[BUGFIX] Followup to "system locale when indexing"

A backport mistake happened letting the jpeg file extension
vanish. Undo that mistake.

Resolves: #30244
Releases: 6.1
Change-Id: I9e12485f03b5f4f468920e47b81308de2e0c6a4a
Reviewed-on: https://review.typo3.org/30897
Reviewed-by: Markus Klein
Tested-by: Markus Klein