The API documentation has this requirement for a REST API request: "For methods that accept request parameters, the platform accepts either application/json or application/x-www-form-urlencoded…

The Accept header 'Accept: application/xml; q=0.5, application/json' tells the server the client prefers application/json (implicit q=1.0) over application/xml (q=0.5), so the server - which only supports returning JSON - will respond with application/json. This is the correct HTTP content negotiation mechanism for controlling the response format. The other entries use Content-Type, which controls the request body format, not the response format.

Content-Type describes the format of the data the client is sending in the request body; it has no effect on the format of the server's response.

Content-Type with an XML preference still controls only the outgoing request body format, and specifying XML would conflict with the server's JSON-only response capability.

The Expect header is used for handshake behaviors such as 100-continue and is not a content negotiation mechanism for specifying response media types.