Again, I'm not especially supportive of the frob as I fail to see very much
utility in it. But I also don't oppose it. I see it as "mostly harmless".
here's the question: say someone else wants to define another frob,
and it's orthogonal to xml. does a type that uses both the new
frob and the xml frob then become application/foo-xml-newfrob
or do MIME readers have to recognize both forms?
what happens when there are three frobs?
can they appear in any order?
if we're going to add frobs to content-types, then it seems like we
should do this in a way that allows for extensibility.