Twitter update-status with media

I have a requirement to update twitter status with media . I am using following Twitter API for it. https://dev.twitter.com/docs/api/1.1/post/statuses/update_with_media.

Doing this with nodejs twit library "https://github.com/ttezel/twit"

twit = new Twit({
                consumer_key:         TWITTER_OAUTH_KEY
              , consumer_secret:      TWITTER_OAUTH_SECRET
              , access_token:         'access token'
              , access_token_secret:  'secret'
            });
wallpost = {};
wallpost.status = 'media upload1 deliverdfd\nhttp://www.animalplanet.com/';
wallpost.media =  [{
                "media_url": "http:\/\/pbs.twimg.com\/media\/A7EiDWcCYAAZT1D.jpg",
                "media_url_https": "https:\/\/pbs.twimg.com\/media\/A7EiDWcCYAAZT1D.jpg",
                "url": "http:\/\/t.co\/bAJE6Vom",
                "display_url": "pic.twitter.com\/bAJE6Vom",
                "expanded_url": "http:\/\/twitter.com\/BarackObama\/status\/266031293945503744\/photo\/1",
                "type": "photo",

            }];

My post request

twit.post( "update_with_media", wallpost,  function(err, res2) {
                if (err) {
                    return next(err);
                }
                console.info(res2);
                // returns the post id
                res.json({
                  status: 200,
                  info: "OK",
                  id: res2.id
                });
            });

Am getting an error message [{"code":195,"message":"Missing or invalid url parameter"}]}. I have googled it found some threads some of them mentioning about set content type multipar form data. But i don't know how to set content type.

I have tried

 var multipart = [{
                "Content-Type": "multipart/form-data"

            }];
wallpost.content_type = multipart;

not working for me. Please help me to solve this issue. Thank you