[BUGFIX] InlineElement: Make file extension check case insensitive

Using an inline element and trying to reference a file with an uppercase
(allowed) file extension does not show an error now.

Resolves: #66167
Releases: master, 6.2
Change-Id: I1da5869f4b1b9543e6afe270bdd3e5bac47b9258
Reviewed-on: http://review.typo3.org/38389


Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>