In order to set the content-type of the http response, you will need to prepend your message content with actual http header for Content-Type. So, you will want to set the content type something like this:
var contentType = "application/json";
var contentTypeLine = "Content-Type: " + contentType + "\r\n";
qie.postMessageResponse(contentTypeLine + message.getNode('/'));
*Note that not all HTTP headers can be set this way.