RFC1521 RFC2045 describes the parsing of the Content-Type field, including optional parameters after the base media type. E.g. following the RFC, a registry could output:

Content-Type: application/vnd.oci.image.index.v1+json; charset=utf-8

Should this be explicitly allowed in the distribution-spec? Currently we say:

If the manifest has a mediaType field, clients SHOULD reject unless the mediaType field's value matches the type specified by the Content-Type header.