multipart/form-data数据格式

 (*
   ***************************************
   ***** multipart/form-data数据格式 *****
   ***************************************

  # 请求头, 这个是必须的, 需要指定Content-Type为multipart/form-data, 指定唯一边界值
  Content-Type: multipart/form-data; boundary=${Boundary}

  # 请求体
  --${Boundary}
  Content-Disposition: form-data; name="name of file"
  Content-Type: application/octet-stream

  bytes of file
  --${Boundary}
  Content-Disposition: form-data; name="name of pdf"; filename="pdf-file.pdf"
  Content-Type: application/octet-stream

  bytes of pdf file
  --${Boundary}
  Content-Disposition: form-data; name="key"
  Content-Type: text/plain;charset=UTF-8

  text encoded in UTF-8
  --${Boundary}--
  *)