[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.

Resolves: #50568
Releases: 6.2, 6.1, 6.0
Change-Id: I5761ab7f06116ef847caeb2ae786e580730c0e6b