Took a look in ~/.config/joplin-desktop/resources/ folder. I have a 58M xxxidstring.crypted file in there, but there is no decrypted file with the same ID. Most of the other .crypted files have a matching decrypted file sitting next to them on the file system.
Is this cruft that just hasn't been cleaned up?
Is there a way to figure out what that encrypted file represents?
It's cache that indeed hasn't been cleaned up. If you close the app, it should be safe to delete all .crypted files, even those with matching resources
And @roman_r_m ... I actually tried that, but as @laurent stated, it is cruft, so my attempt to link it led to nothing (no decrypted version with that ID).