[!!!][TASK] Convert thumbnails only for non-image files

The global option "thumbnails_png" must only be taken
into account if the processed file is not an image.

Resolves: #73106
Releases: master
Change-Id: I9d806631331924f680d4d7834136e6860c6d95d8
Reviewed-on: https://review.typo3.org/46480


Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>