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).
This reverts commit 7360db05b4.
This commit was added as an attempt to fix#6046. Unfortunately, the fix
is completely wrong. As reported on #10359, embedded thumbnails are not
displayed in VLC, and Se7en on IRC reports that the embedded thumbnail
misleads mpv as well.
The correct way is using -attachment of ffmpeg, while the current
run_ffmpeg_multiple_files API can't handle it cleanly.
