This is perhaps a bit silly, but I figure it shouldn't be too hard to implement: When viewing one of the thumbnail cache folders like `$XDG_CACHE_HOME/thumbnails/normal`, thumbnails are not generated for obvious circularity reasons. However, we could treat those files as their own thumbnails: when the `normal` sized thumbnail is requested for `$XDG_CACHE_HOME/thumbnails/normal/hash.png`, tumbler could simply return `$XDG_CACHE_HOME/thumbnails/normal/hash.png`.