C# HttpWebRequest 上传图片的有关问题

C# HttpWebRequest 上传图片的问题
C#语言 winform 做 模拟登陆到阿里巴巴国际站进行产品的批量发布, 发布产品图片上传返回的是{"code":"1"}上传失败,直接登入速卖通发布的时候 通过httpwatch抓取返回的内容是:{"height":"762","hash":"b46dfd6df231a5ae03df29854da57ee2","width":"1000","code":"0","fs_url":"/common/upload/140/193/039/375/hz-fileserver-upload-05_hw1d3fup.jpg","url":"http://upload.alibaba.com/common/upload/140/193/039/375/hz-fileserver-upload-05_hw1d3fup.jpg?size=100487&height=762&width=1000&hash=b46dfd6df231a5ae03df29854da57ee2","size":"100487"}
流的内容:

以下是我的代码：
通过webbrowser 获取cookie ：
public CookieContainer getCookieContainer(WebBrowser webBrowser1)
        {
            CookieContainer myCookieContainer = new CookieContainer();
            string cookieStr = webBrowser1.Document.Cookie;
            string[] cookstr = cookieStr.Split(';');
            foreach (string cookie in cookstr)
            {
                string name  = cookie.Split('=')[0];
                string value = cookie.Substring(name.Length + 1); 
                string domain = "";
                if (name.Trim() == "me_p_info")
                {
                    domain = ".posting.aliexpress.com";
                }
                else if (name.Trim() == "JSESSIONID")
                {
                    domain = ".posting.aliexpress.com";
                }
                else {
                    domain = ".aliexpress.com";
                }
                string path = "/"; 
                myCookieContainer.Add(new Cookie(name.Trim(), value.Trim(), path, domain));
            }
            return myCookieContainer;
        }
HttpWebRequest  设置 cookie 且post 提交 

 public string createFilePost(string file, string url, CookieContainer cookieContainer)
        {
            DateTime timeStamp = new DateTime(1970, 1, 1); //得到1970年的时间戳 
            long a = (DateTime.UtcNow.Ticks - timeStamp.Ticks) / 10000; //注意这里有时区问题，用now就

            string httpWebContent = string.Empty;
            string ksuploadboundary = "----ksuploadboundary" + a.ToString();
            HttpWebRequest uploadRequest = (HttpWebRequest)WebRequest.Create(url);//url为上传的地址
            uploadRequest.ContentType = "multipart/form-data;boundary=" + ksuploadboundary;
            uploadRequest.Method = "POST";
            uploadRequest.Accept = "*/*";
            uploadRequest.KeepAlive = true;
            uploadRequest.Headers.Add("Accept-Language","zh-CN");
            uploadRequest.Headers.Add("Accept-Encoding", "gzip,deflate");
            uploadRequest.Referer = "http://posting.aliexpress.com/style/js/post/app/editor/uploader/flash.swf";