code: private void sendMultipart() throws IOException {
  		setMultipart();// 设置表单类型为Multipart
  
  		final OutputStream out = this.httpConnection.getOutputStream();
  		try {
  			writeFileForm(out);
  			writeForm(out);
  			formEnd(out);
  		} catch (IOException e) {
  			throw e;
  		} finally {
  			IoUtil.close(out);
  		}
  	}
docs: 发送多组件请求（例如包含文件的表单）
  
  @throws IOException