[FEATURE] Show allowed file extensions in FAL field

Above the list of file relations are buttons to add or upload new images.
Now, behind these buttons a label appears, which shows the allowed file
extensions to upload/use.

Resolves: #45428
Releases: master
Change-Id: I6e7caea22c6198819084db4c737088cc6f7eb02f
Reviewed-on: http://review.typo3.org/36443


Reviewed-by: Benjamin Kott <info@bk2k.info>
Tested-by: Benjamin Kott <info@bk2k.info>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>