protected function loadConfigFile(string $key)
    {
        $key = strtolower($key);
        $filename = $this->path . DIRECTORY_SEPARATOR . ucfirst($key) . Config::CONFIG_FILE_EXTENSION;

        if (file_exists($filename)) {
            $this->configuration[$key] = require $filename;
            return;
        }

        throw new ConfigException('Unable to load resource: ' . $filename);
    }