Elgg\Application::start PHP Method

    public static function start()
    {
        $app = self::create();
        $app->bootCore();
        return $app;
    }

 /**
  * Handle a request for a file
  *
  * @param Request $request HTTP request
  * @return Response
  */
 public function getResponse($request)
 {
     $response = new Response();
     $response->prepare($request);
     $path = implode('/', $request->getUrlSegments());
     if (!preg_match('~download-file/g(\\d+)$~', $path, $m)) {
         return $response->setStatusCode(400)->setContent('Malformatted request URL');
     }
     $this->application->start();
     $guid = (int) $m[1];
     $file = get_entity($guid);
     if (!$file instanceof ElggFile) {
         return $response->setStatusCode(404)->setContent("File with guid {$guid} does not exist");
     }
     $filenameonfilestore = $file->getFilenameOnFilestore();
     if (!is_readable($filenameonfilestore)) {
         return $response->setStatusCode(404)->setContent('File not found');
     }
     $last_updated = filemtime($filenameonfilestore);
     $etag = '"' . $last_updated . '"';
     $response->setPublic()->setEtag($etag);
     if ($response->isNotModified($request)) {
         return $response;
     }
     $response = new BinaryFileResponse($filenameonfilestore, 200, array(), false, 'attachment');
     $response->prepare($request);
     $expires = strtotime('+1 year');
     $expires_dt = (new DateTime())->setTimestamp($expires);
     $response->setExpires($expires_dt);
     $response->setEtag($etag);
     return $response;
 }