public function registerAlias($alias, $file, $extension = null)
    {
        if ($extension === null) {
            $extension = File::extension($file);
        }

        $extension = strtolower($extension);

        if (!isset($this->aliases[$extension])) {
            $this->aliases[$extension] = [];
        }

        $this->aliases[$extension][$alias] = $file;

        return $this;
    }