code: func PostFile(fieldname, filename, uri string) ([]byte, error) {
  	fields := []MultipartFormField{
  		{
  			IsFile:    true,
  			Fieldname: fieldname,
  			Filename:  filename,
  		},
  	}
  	return PostMultipartForm(fields, uri)
  }
docs: //PostFile 上传文件