/**
 * Implements hook_file_url_alter().
 */
function {{ machine_name }}_file_url_alter(&$uri) {
  $user = \Drupal::currentUser();

  // User 1 will always see the local file in this example.
  if ($user->id() == 1) {
    return;
  }

  $cdn1 = 'http://cdn1.example.com';
  $cdn2 = 'http://cdn2.example.com';
  $cdn_extensions = ['css', 'js', 'gif', 'jpg', 'jpeg', 'png'];

  // Most CDNs don't support private file transfers without a lot of hassle,
  // so don't support this in the common case.
  $schemes = ['public'];

  /** @var \Drupal\Core\StreamWrapper\StreamWrapperManagerInterface $stream_wrapper_manager */
  $stream_wrapper_manager = \Drupal::service('stream_wrapper_manager');

  $scheme = $stream_wrapper_manager::getScheme($uri);

  // Only serve shipped files and public created files from the CDN.
  if (!$scheme || in_array($scheme, $schemes)) {
    // Shipped files.
    if (!$scheme) {
      $path = $uri;
    }
    // Public created files.
    else {
      $wrapper = $stream_wrapper_manager->getViaScheme($scheme);
      $path = $wrapper->getDirectoryPath() . '/' . $stream_wrapper_manager::getTarget($uri);
    }

    // Clean up Windows paths.
    $path = str_replace('\\', '/', $path);

    // Serve files with one of the CDN extensions from CDN 1, all others from
    // CDN 2.
    $pathinfo = pathinfo($path);
    if (isset($pathinfo['extension']) && in_array($pathinfo['extension'], $cdn_extensions)) {
      $uri = $cdn1 . '/' . $path;
    }
    else {
      $uri = $cdn2 . '/' . $path;
    }
  }
}