We don't seem to understand what .vtt is.
Created attachment 351699 [details] Patch I got the MIME lists from `MIMETypeRegistry.cpp`. There is some overlap, so I'm not sure how we want to deal with that, but this is a step in the right direction.
<rdar://problem/45056079>
Comment on attachment 351699 [details] Patch r=me
Comment on attachment 351699 [details] Patch Clearing flags on attachment: 351699 Committed r236892: <https://trac.webkit.org/changeset/236892>
All reviewed patches have been landed. Closing bug.