gallery mode: images only?
The gallery mode is useful but it can't exclude other files than images. would an option to check for the mime type be too heavy?
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 File:Opampinstrumentation.xcf 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).
