I can't tell exactly what you are trying to do in this PR. The goal of this formatter is to add it to the bottom of the list and allow the default JSON formatter to run for all other requests, allowing this one to handle requests with a correct Accept header for valid or often used JSONP media types. Earlier attempts tried to allow the same formatter to be used to service all JSON and JSONP requests, but there are both security vulnerabilities and other issues I've since forgotten, some having to do with mismatched Content-Type headers.