Custom Thumbnails for Files?

In that case, Opus 13's "thumbnail styles" might be all you need. You can specify that folder thumbnails look for a named file inside the folder and then (optionally) combine that file with another image to produce the folder thumbnail. Example.

In Opus 12, you could also name the thumbnail folder.jpg to do something similar, but with the image inside a standard folder frame/background, rather than a custom (or removed) frame/background. That also works with the Windows shell, so you'd see the thumbnails in File Open dialogs, although the way the shell renders them leaves something to be desired. (Depending on Opus configuration, Opus can render those folder thumbs via the shell, or by itself. You'd probably want the latter to avoid the image being distorted or partially covered.)

Just to reiterate, the files themselves would not have special thumbnails; only the folders they are in.

Changing the thumbnails for the files themselves would require writing some plugin code. But, thinking on it some more, it could be fairly simple code if the aim is just to e.g. use filename_thumb.png as the thumbnail for (And you could hide the *_thumb.png files so they don't clutter things up. Or keep them in a separate directory somewhere.) A plugin to do that would be quite simple, and we can definitely help there if needed.