Poster attribute only allows core media type

[mattgarrish]

Another weird test/restriction I've come across is in content-publication.feature where the "Report a poster attribute that references an invalid media type" scenario doesn't recognize a manifest fallback. Even if I add a fallback to a CMT, I get the MED-001 error.

Manifest fallbacks may not work in reading systems, but I don't see the difference between this case and, say, img/src referencing an foreign resource.

[rdeltour]

Agreed, this looks like a bug. I'm also wondering why we have a special check message for poster 🤔. MED-001 may disappear altogether when I fix this.