post 请求数据返回 Unsupported Media Type

public static string GetPostData(string url, string paramstr)
        {
            HttpWebRequest webrequest = (HttpWebRequest)HttpWebRequest.Create(url);
            webrequest.Method = "post";
            webrequest.ContentType = "application/json;charset=UTF-8";

            byte[] postdatabyte = Encoding.UTF8.GetBytes(paramstr);
            webrequest.ContentLength = postdatabyte.Length;
            Stream stream;
            stream = webrequest.GetRequestStream();
            stream.Write(postdatabyte, 0, postdatabyte.Length);
            stream.Close();



            using (var httpWebResponse = webrequest.GetResponse())
            using (StreamReader responseStream = new StreamReader(httpWebResponse.GetResponseStream()))
            {

                String ret = responseStream.ReadToEnd();



                return ret;
            }
        }