[BUGFIX] Properly trimExplode allowed file extensions in IRRE / FAL

When a custom FAL / sys_file_reference inline element is registered, and
there are custom allowed file extensions, the property "elementBrowserAllowed"
is not properly run through "GeneralUtility::trimExplode()", however, all other
places do this properly.

The changes moves the trimExplode to the correct place, so uploading
of files directly in FormEngine works properly.

Resolves: #89540
Releases: master, 9.5
Change-Id: I9a486f8c932078629761bd86cc1d9455120fc7a5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62155


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Gorges <daniel.gorges@b13.com>
Tested-by: Susanne Moog <look@susi.dev>
Reviewed-by: Daniel Gorges <daniel.gorges@b13.com>
Reviewed-by: Susanne Moog <look@susi.dev>