[BUGFIX] Ignore case in file extension filter

This patch makes the FileExtensionFilter case insensitive.

If the filter is configured to allow files with the ".EXT"
extension it will now also allow files with an ".ext"
extension.

Additionally the order of a data provider method in the
FileExtensionFilterTest has been corrected.

Resolves: #50568
Releases: 6.2, 6.1, 6.0
Change-Id: I5761ab7f06116ef847caeb2ae786e580730c0e6b
Reviewed-on: https://review.typo3.org/22724
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters