// Build the trailing boundary string as a byte array
// ensuring the boundary appears on a line by itself
byte[] boundaryBytes = Encoding.ASCII.GetBytes("\r\n--" + boundary + "\r\n");
FileStream fileStream = new FileStream(fileName, FileMode.Open, FileAccess.Read);
httpWebRequest2.ContentLength = fileStream.Length + (long)postHeaderBytes.Length + (long)boundaryBytes.Length; //一定需要长度吗?
// Write out the trailing boundary