HTTP POST request can not support JSONP.
Here is some resource :
1) Cross-domain AJAX with JSONP
Then the next limitation — and this is a big one — is that JSONP doesn’t support POST requests.
2) JSONP in Jquery.POST()
3) Using PUT/POST/DELETE with JSONP and jQuery
4) Ext.util.JSONP POST possible?
So It need change the REST service from POST to GET. but Get has message size limitation. Another way is user server side proxy or other solution .