[BUGFIX] Enable thumbnail generation for IRRE image files with capitalized file extension

The 'imagefile_ext' list only contains the lowercase versions of all supported extensions.
This patch makes sure that a lowercase version of the file extension is used and it is
recognised as an image file.

Resolves: #81826
Releases: master, 8.7
Change-Id: Ice5881be4c685432987530dcdd50b1f758d883ef
Reviewed-on: https://review.typo3.org/53427


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>