[BUGFIX] Make FileExtensionFilter case insensitive

Resolves: #55359
Resolves: #55614
Releases: 6.2
Change-Id: I8aceb208ba2f28967f08e9ac458d9a9da3de507b
Reviewed-on: https://review.typo3.org/27359
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters