关于HttpWebRequest上传图片的问题

请求报文如下

C#代码

     static void Main(string[] args)
        {
            string boundary = "----" + DateTime.Now.Ticks.ToString("x2");

            StringBuilder sb = new StringBuilder();
            sb.Append("--" + boundary + "\r\n");
            sb.AppendFormat("Content-Disposition: form-data; name=\"file\"; filename=\"{0}\"\r\n", "888.jpg");
            sb.Append("Content-Type: image/jpeg\r\n\r\n");

            //sb.Append("--" + boundary + "--");  //加了这句服务端图片显示不了

            HttpWebRequest req = (HttpWebRequest)WebRequest.Create("http://localhost:6743/upload.ashx");
            req.ContentType = "multipart/form-data; boundary=" + boundary;
            req.Method = "POST";

            Stream reqStream = req.GetRequestStream();

            reqStream.Write(Encoding.UTF8.GetBytes(sb.ToString()),0, Encoding.UTF8.GetBytes(sb.ToString()).Length);

            FileStream fileStream=new FileStream("888.jpg", FileMode.Open, FileAccess.Read);

            byte[] buffer = new byte[(int)fileStream.Length];

            int bytecount = 0;
            while ((bytecount=fileStream.Read(buffer,0,buffer.Length))!=0)
            {
                reqStream.Write(buffer,0,bytecount);
            }

            byte[] boundarybytes = System.Text.Encoding.ASCII.GetBytes("\r\n--" +
            boundary + "\r\n");
            reqStream.Write(boundarybytes, 0, boundarybytes.Length);
          

            reqStream.Dispose();


            string ssss=new StreamReader(req.GetResponse().GetResponseStream()).ReadToEnd();

            Console.ReadKey();

        }

我想问的问题就是，请求报文里圈出来的那个 要不要用 stringbuilder 加入进去，我不加入的话服务端可以接收到文件 并成功打开，
但加入的话 服务端可以接收到文件，但无法打开图片