The code currently hardcodes `text/srt` for all subtitles.
`text/srt` is wrong, it seems `application/x-subrip` is the official
extension coming from the official mime database, at least (and still
works with the Samsung TV I tested with). Also add that one to `fs/
mimetype.go`.
Compared to previous iterations of this PR, I dropped tests ensuring
certain mime types are present - as detection still seems to be fairly
platform-specific.