public function unregisterFiles(PortableElementObject $object, $files)
    {
        $deleted = true;
        $filesystem = $this->getFileStorage();
        foreach ($files as $relPath) {
            $fileId = $this->getPrefix($object) . $relPath;
            if (!$filesystem->has($fileId)) {
                throw new \common_Exception('File does not exists in the filesystem: ' . $relPath);
            }
            $deleted = $filesystem->delete($fileId);
        }
        return $deleted;
    }