class RSSEnclosureFormatter

class RSSEnclosureFormatter extends FileFormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $entity = $items->getEntity();
        // Add the first file as an enclosure to the RSS item. RSS allows only one
        // enclosure per item. See: http://wikipedia.org/wiki/RSS_enclosure
        foreach ($this->getEntitiesToView($items, $langcode) as $file) {
            
            /** @var \Drupal\file\FileInterface $file */
            $entity->rss_elements[] = [
                'key' => 'enclosure',
                'attributes' => [
                    // In RSS feeds, it is necessary to use absolute URLs. The 'url.site'
                    // cache context is already associated with RSS feed responses, so it
                    // does not need to be specified here.
'url' => $file->createFileUrl(FALSE),
                    'length' => $file->getSize(),
                    'type' => $file->getMimeType(),
                ],
            ];
        }
        return [];
    }

}