Prevent image thumbnailer to take over raw images when raw thumbnailer has been disabled.
There is a separate thumbnailer for RAW image files. This fixes the problem that the generic image thumbnailer kicks in for some RAW file types eventhough RAW thumbnailing has been disabled in Dolphin by the user.
Note: Not completely sure whether excluding "image/*" removes thumbnail generation for some image formats that are not explicitly specified/ suppported somewhere else. At quick glance it looks ok: All image formats I can think of seem to be handled by other thumbnailers (e.g. there's a separate JPEG thumbnailer) or by image thumbnailer itself.
Fixes BUG: 460723