support appimages in disk-image-mounter
specs https://github.com/AppImage/AppImageSpec
mime type
application/x-iso9660-appimage
https://cgit.freedesktop.org/xdg/shared-mime-info/commit/?id=01fa61fc002afdcf43f61e7df2d6cc6f6968d8d2
in short, they are two types Type 1: zisofs based Type 2: squashfs based
the older "zisofs" type already just works if the extension is changed to iso. "squashfs" has an offset, and doesn't work in current image-mounter.