public function refresh()
    {
        $assetClassType = str_replace('Neos\Media\Domain\Model\\', '', get_class($this));
        $this->systemLogger->debug(sprintf('%s: refresh() called, clearing all thumbnails. Filename: %s. PersistentResource SHA1: %s', $assetClassType, $this->getResource()->getFilename(), $this->getResource()->getSha1()));

        // whitelist objects so they can be deleted (even during safe requests)
        $this->persistenceManager->whitelistObject($this);
        foreach ($this->thumbnails as $thumbnail) {
            $this->persistenceManager->whitelistObject($thumbnail);
        }

        $this->thumbnails->clear();
    }