function downloadFile() {
  // Download a file now (GET), so we can upload it in the HTTP POST below.
  var response = UrlFetchApp.fetch('https://www.google.com/humans.txt');
  var fileBlob = response.getBlob();

  var payload = {
    fieldOne: 'value for field one',
    fieldTwo: 'value for field two',
    fileAttachment: fileBlob,
  };

  // Because payload is a JavaScript object, it will be interpreted as
  // an HTML form. (We do not need to specify contentType; it will
  // automatically default to either 'application/x-www-form-urlencoded'
  // or 'multipart/form-data')

  var options = {
    method: 'post',
    payload: payload,
  };

  UrlFetchApp.fetch('http://example.com/upload_form.cgi', options);
}