[BUGFIX] Missing file extension class with "file links" type

The content element type "file links" can't render
the the css class for the file extension, because
it uses a non-existing reference from pre-FAL
times. You can find the specific css class on the
generated list item.

Fixes: #48996
Releases: 6.2, 6.1, 6.0
Change-Id: I3031770a6741a331647f3bd5699ea1a2c0d444a9
Reviewed-on: https://review.typo3.org/21318
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters