Well, the mime type of your resource seems to be missing, how is the getContentType() method from UserResource implemented?
i have this for contentType. But the class is never instantiated, constructor is never called and getContentType() is never called.
Ok, you need a set mime-type in your web.xml, just like the error message tells you.
It should look like this (taken from the web.xml of the sandbox project)
Oh, now it works! Thank you Michal.