public function loadBulk(array $bulk)
    {
        foreach ($bulk as $user => $files) {
            if (!is_string($user)) {
                throw new \InvalidArgumentException();
            }

            if (!is_array($files) && !is_string($files)) {
                throw new \InvalidArgumentException();
            }

            $files = (array) $files;
            foreach ($files as $file) {
                $this->load($user, $file);
            }
        }
        return $this;
    }