well there is a no "standard" for extracting version information from a media type. at the same time there can of course be multiple media types in the Accept header and its the job of the negotiation to figure out which one to use. however in the end we only know the format and not the actual media type (though this might be fixable).

that being said, in 95% of the cases when it comes to using versioned media types, people are passing in a single media type anyway.

/cc @willdurand @adrienbrault