Validation fails if artifact.mime_type is not IANA registered mime-type

[tlhampton13]

The 2.1 spec only recommends artifact.mime_type be IANA registered, however the validator has a MUST check that requires the mime_type to be IANA registered (musts.artifact_mime_type).

This check should be removed from the MUST checks.

Not all mime types in the wild are registered but are very much of interest. It needs to be possible to express these unregistered types in STIX and also pass validation.