msgpack seems to be an extremely fast, if extremely new format for data serialisation. Does it have a recognised MIME type yet? If not, what should be used in the interim?
From Wikipedia : According to RFC 6838 (published in January 2013 : http://tools.ietf.org/html/rfc6838), any use of types in the "x." tree is strongly discouraged. Media types with names beginning with "x-" are no longer considered to be members of this tree since January 2013.
Then use directly application/msgpack