parameter for content-type

[pdowler]

In DataLink, we specify a parameter to add to the VOTable mime type to tell clients that the table contains "datalinks":

application/x-votable+xml; content=datalink

Over in the DataLink discussions about adding params to other content-type values (eg in the content_type column), we found out that this really goes against the spirit of the mime type RFC 2045 and that params should be defined by the spec that owns the type. So, this is a request for VOTable to add an allowed mime type parameter content that other specs can define values for. It would thus be open ended but clients would know to anticipate the param in some cases.