Ok, so I've found the answer after diving into the java sources for the rest service.
The RestServiceRequest class has the following method:
private RequestContentType determineRequestContentType(HttpServletRequest request)
This method is only able to detect JSON content if the Content-Type header for the request contains "text/json".
Since I used "application/json" as Content-Type this obviously didn't work, the content-type was internally set to 'OTHER' and it couldn't automatically parse the JSON I send.
Hi Thomas,
You should definitly post so much data in the body of the request. But note that this doesn't depend on the published service, but on the client you are using to make the call. The published service is capable of both handling GET and POST requests. I don't know how you are making the call currently, but you should change something there.
Note that the REST module currently doesn't accept XML as input data. If you want to send XML to a Mendix app, just use webservices, the REST module only accepts json, binary, multipart or formdata.
Update
Looks fine. Note that the attribute names are case sensitive. Also, is the attribute visible in the microflow from security perspective?