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).
7ec1e417ca