7ec1e417ca
To fix rg3#10359, use `-attach` to embed the thumbnail as attachment. In this version, the attached filename follow the https://matroska.org/technical/cover_art/index.html convention, as pointed in #6046. The image dimensions are probably not going to respect the convention though, and we assume the thumbnail is in landscape orientation. As only 2 images type seems to be recognized by the convention, they are hardcoded (mimetypes module would match on extension too).