app.get('/download', function(req, res, next) {
  // Get the download sid
  var downloadSid = req.query.sid;

  // Get the download file path
  getDownloadFilePath(downloadSid, function(err, path) {
    if (err) return res.end('Error');

    // Read and send the file here...

    // Finally, delete the download session to invalidate the link
    deleteDownload(downloadSid, function(err) {
      // ...
    });
  });
});