Thanks @Rick Weyrauch
This is a x-request-execution-correlation-id:
Edit: I located the origin of the issue. The informative message packed at the response object did make sense; I was passing the wrong header to the request and thus the request was indeed malformed. Just FYI, this error status code can be cause for a missing or wrong specified content-type at the headers.
You can get a full list of all possible HTTP Status Codes here
You should be receiving a body with an error message telling you why you are receiving a "415 Unsupported Media Type".
Under the OData spec, not all MIME types are supported, but I will have to check on what we publish past referencing the OData Spec.
Your reply should also contain a X-Request-Execution-Correlation-Id HTTP header. This is your "call receipt" and can be used to ask "What happened on this specific call?" type questions. If you can provide me that id, I can look up your specific request.