So when attaching files to a matroska container they get a name, normally their original name. However when you are running windows and you supply the full path to the file you want to attach, ffmpeg sets that full path as name of the attachment. How can I rename that attachment, preferably in the same command as the attachment process?
Ok, months later I know the solution: Use
-metadata:s:t filename="cover.jpg"
Replace cover.jpg with your desired filename to be stored in the matroska file. If you run more complex stuff with multiple attachments the stream specifier (:s:t) might be different for you, see -map+metadata ffmpeg documentation