public function filterByExtension($extension)
    {
        $extensions = is_array($extension) ? $extension : [$extension];
        if (count($extensions) > 0) {
            $expr = $this->query->expr();
            $or = $expr->orX();
            foreach ($extensions as $extension) {
                $extension = ltrim((string) $extension, '.');
                $or->add($expr->eq('fv.fvExtension', $this->query->createNamedParameter($extension)));
                if ($extension === '') {
                    $or->add($expr->isNull('fv.fvExtension'));
                }
            }
            $this->query->andWhere($or);
        }
    }