code: def send_file_url(self, recipient_id, file_url, notification_type=NotificationType.regular):
          
          return self.send_attachment_url(recipient_id, "file", file_url, notification_type)
docs: Send file to the specified recipient.
          https://developers.facebook.com/docs/messenger-platform/send-api-reference/file-attachment
          Input:
              recipient_id: recipient id to send to
              file_url: url of file to be sent
          Output:
              Response from API as <dict>