Neos\Neos\Controller\Backend\ContentController::assetsWithMetadataAction PHP Method

    public function assetsWithMetadataAction(array $assets)
    {
        $this->response->setHeader('Content-Type', 'application/json');
        $result = array();
        foreach ($assets as $asset) {
            $result[] = $this->getAssetProperties($asset);
        }
        return json_encode($result);
    }