Accept: application/json

I'm currently using header('Content-type: application/json'); Source: http://stackoverflow.com/questions/267546/correct-http-heade...

I think they want it so that if they send an Accept header in the request that asks for json, you reply with json, instead of using a query parameter to specify the format.

https://developer.mozilla.org/en/HTTP/Content_negotiation has more details about the accept header and its use.