c# image 转 Base64 post 请求 “+” 消失（转 空格）问题

 //图片转base64

public static String ImageFile2Base64(String imageFile)
        {
            Image image = Image.FromFile(imageFile);
            MemoryStream ms = new MemoryStream();
            image.Save(ms, image.RawFormat);
            byte[] byteArray = ms.ToArray();
            ms.Close();
            return "data:image/"+ 文件格式 +";base64," + Convert.ToBase64String(byteArray);
        }

//base64  做UrlEncode 转换
 public static string UrlEncode(string str)
        {
            StringBuilder sb = new StringBuilder();
            byte[] byStr = System.Text.Encoding.UTF8.GetBytes(str); 
            for (int i = 0; i < byStr.Length; i++)
            {
                sb.Append(@"%" + Convert.ToString(byStr[i], 16));
            }

            return (sb.ToString());
        }

   postData += "&img=" + ImageData.UrlEncode(imgfile);
   byte[] data = Encoding.UTF8.GetBytes(postData);