AV_DISPOSITION_ATTACHED_PIC usually means the video track isn't real,
and merely reflects the presence of an embedded image in tag data (such
as ID3v2 tags), with some inconsistent hack to make libavformat return
it as video packet once.
Except it doesn't mean that. It can be randomly set on other streams
that do sort of behave like video streams, such as chapter thumbnail
tracks in mp4 files. AV_DISPOSITION_TIMED_THUMBNAILS is set in these
cases. In theory, there can supposedly be more such cases, but only the
chapter thumbnail one currently exists. So add it as exception.
This restores displaying these thumbnails as video frames, for better or
worse. (Before, only the first thumbnail was displayed.)
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