twp: add jxl, avif, and webp as allowed wallpaper mimemtypes

Fedora wants to use jxl for wallpapers to reduce installation footprint (https://fedoraproject.org/wiki/Changes/SwitchToJXLforDefaultWallpaper). As discussed in https://pagure.io/fesco/issue/3376#comment-961826, we want thunar to allow jxl and the other newer graphics formats.