The MIME type is allowed to add information about the nature of the data encoded on top of the encoding. This is why we have application/xhtml+xml. It means "this is encoded using xml, and btw the parsed xml structure is xhtml".
The ideal place to handle this is (as pointed out above) content negociation - specifically as 'application/rjson+json'
The ideal place to handle this is (as pointed out above) content negociation - specifically as 'application/rjson+json'