protected static function normalizeNested(array $files)
    {
        $normalized = [];
        foreach (array_keys($files['tmp_name']) as $key) {
            $spec = [
                'name'     => $files['name'][$key],
                'tmp_name' => $files['tmp_name'][$key],
                'type'     => $files['type'][$key],
                'size'     => $files['size'][$key],
                'error'    => $files['error'][$key],
            ];
            $normalized[$key] = static::build($spec);
        }

        return $normalized;
    }