gallery mode: images only?
Compared to the other things this extension does, its probably would not be horrible. Just doing filter by mime type wouldn't really filter out all non-images since images can cover multiple mime types (although filtering by mime type is just as easy as doing show only stuff that is image/*, but that still brings up the question of what is an image. Is a pdf an image, it can be displayed as an image with a thumbnail. Is an xcf file an image? it certainly is, but files like cannot be displayed as a thumbnail. (otoh, if you just want one specific type of image, the what is an image debate is kind of moot).