The Google Script will automatically download a file from the Internet and upload it to another HTML form with an HTTP POST request. If the file already exists in your Google Drive, you can use the DriveApp.getFileById() method to retrieve the file and send it to the web form. Also see: Receive Files in Google Drive

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);
}