未知扩展名图片下载

/// <summary>
        /// 抓取远程图片 自动识别扩展名
        /// </summary>
        /// <param name="url"></param>
        /// <returns></returns>
        private string getRemoteImg(string url)
        {
            string rtnString = string.Empty;
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
            request.Method = "GET";
            try
            {
                using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
                {
                    string fileExt = string.Empty;

                    string fileName = DateTime.Now.ToString("yyyyMMddhhmmssss");
                    string FileExtNum = string.Empty;
                    using (Stream stream = response.GetResponseStream())
                    {
                        byte[] retBytes = null;
                        using (MemoryStream ms = new MemoryStream())
                        {
                            int b;
                            int i = 0;
                            while ((b = stream.ReadByte()) != -1)
                            {
                                i++;
                                if (i <= 2)
                                {
                                    FileExtNum += b.ToString();
                                }
                                ms.WriteByte((byte)b);
                            }
                            retBytes = ms.ToArray();
                        }
                        switch (FileExtNum)
                        {
                            case "255216":
                                fileExt = "jpg";
                                break;

                            case "7173":
                                fileExt = "gif";
                                break;

                            case "13780":
                                fileExt = "png";
                                break;

                            case "6677 ":
                                fileExt = "bmp";
                                break;
                        }
                        if (!string.IsNullOrEmpty(fileExt))
                        {

                            if (!Directory.Exists(Application.StartupPath + "\\UploadFiles\\"))//若文件夹不存在则新建文件夹   
                            {
                                Directory.CreateDirectory(Application.StartupPath + "\\UploadFiles\\"); //新建文件夹   
                            }
                            string localPath = Application.StartupPath + "\\UploadFiles\\" + fileName + "." + fileExt + "";
                            using (FileStream fs = new FileStream(localPath, FileMode.Create))
                            {
                                using (BinaryWriter sr = new BinaryWriter(fs))
                                {
                                    sr.Write(retBytes, 0, retBytes.Length);
                                    sr.Close(); fs.Close();
                                    if (System.IO.File.Exists(localPath))
                                    {
                                        rtnString = "UploadFiles/" + fileName + "." + fileExt + "";
                                    }
                                }
                            }
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                throw ex;
            }
            return rtnString;
        }