Hey there folks, hope you're well.
I have a music collection with folder.jpg files in each folder containing coverart. This coverart shows perfectly in the thumbnail view for folders with 2 music files or more, but not for folders with a single audio file (e.g. CD singles).
Any ideas?
Fotis
Putting folder.jpg in Preferences / Miscellaneous / Advanced [Cosmetic]: cd_thumb_coverart_file might help.
Copy it to coverart.jpg if you want to make it explicit that it should be displayed as a CD album thumbnail. Otherwise it only happens if the folder contains mostly music files, which won’t trigger with just one music file.
Thank you both so much for the replies. I went with Leo's method and it all works beautifully! 
Is it possible to have multiple names and wildcards in Preferences / Miscellaneous / Advanced [Cosmetic]: cd_thumb_coverart_file
like folder.jpg|coverart.jpg|etc.*
It can only be a single name.
You wouldn't normally need more than one name, and you almost definitely wouldn't want to make Folder.jpg turn into a CD jewel case, as that would make lots of folders that don't contain music look like CDs.
Folder.jpg is used by other folders to set a simple image as their thumbnail, and also works in File Explorer. Opus will make it look like a CD jewel case if the folder it's for has mostly music under it, but does something similar to File Explorer in other cases. (That can also be turned off, if it causes problems and you only want coverart.jpg to look like a CD.)
Have had instances of 'cover.jpg' and PNG coverart.
Renaming/converting the files is easier than adding the names to a big list. Also faster as a big list of files would mean Opus has to check if more files exist each time it makes a folder thumbnail.
We do support folder.png, I think. We could add support for coverart.png but I don't think anyone's ever asked for it before (and specifying it as the cover-art filename should work; AFAIK it doesn't need to be a .jpg name).