[BUGFIX] ArrayIterator::seek() warning in ElementBrowser

ElementBrowser calls Folder::getFiles() with wrong parameters.
Properly implement the file extensions filter.

Resolves: #51752
Releases: 6.2, 6.1, 6.0
Change-Id: I56468c79225e2d3baa5e5784571074532e2287ad
Reviewed-on: https://review.typo3.org/26476
Reviewed-by: Markus Klein
Tested-by: Markus Klein