I have tried to do a fair amount of debugging/searching, but I can’t figure out why the thumbnail images are not showing in the Cosmic Files app for png or jpg files in the Fedora Atomic Cosmic rawhide image I’m using. It seems to work on AerynOS’s version of the Cosmic desktop which is also on Cosmic’s beta.6. So not sure why it isn’t working (at least for me) with Fedora Cosmic Atomic.
[SOLVED- this is not a specific Fedora Cosmic Atomic issue]. I just figured out this issue only relates to my fuse.rclone mounted GoogleDrive directory. If I copy images from there into my normal ~/Downloads folder, Cosmic File Manager happily shows the thumbnails. It must be a (probably sensible) policy decision not to support thumbnails over fuse mounted/network directories.