Drupal 8 viewer.js file formatter for embedded pdf previews

ViewerJsFormatter.php

<?php

/**
 * @file
 * Contains Drupal\viewerjs\Plugin\Field\FieldFormatter\ViewerJsFormatter.
 *
 * Download the ViewerJS library 
 * at https://github.com/abuhamza/Viewer.js/archive/master.zip 
 * and extract it to libraries/viewerjs
 * (so that the viewer.js will be located at 
 * http://example.com/libraries/viewerjs/viewer.js)
 */

namespace Drupal\viewerjs\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FormatterBase;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Url;


/**
 * Plugin implementation of the 'viewer_js' formatter.
 *
 * @FieldFormatter(
 *   id = "viewerjs",
 *   module = "viewerjs",
 *   label = @Translation("ViewerJS Previewer"),
 *   field_types = {
 *     "file", "image"
 *   }
 * )
 */
class ViewerJsFormatter extends FormatterBase {

  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    foreach ($items as $delta => $item) {
      $filename = $item->entity->getFilename();
      $splited_uri = explode('.', $filename);
      $extension = end($splited_uri);
      $viewerjs_path = 'libraries/viewerjs';
      $download_url = file_create_url($item->entity->getFileUri());
      $path = '/' . $viewerjs_path . '/' . $this->getSetting('template') . '#' . $download_url;

      if (in_array(strtolower($extension), $this->viewerjsSupportedExtensions())) {
        $elements[$delta]['preview'] = [
          '#markup' => '<iframe src = "'.$path.'" width="400" height="300" allowfullscreen webkitallowfullscreen></iframe>',
          '#allowed_tags' => ['iframe'],
        ];
      }
      $elements[$delta]['link'] = [
        '#type' => 'link',
        '#title' => $filename,
        '#url' => Url::fromUri($download_url),
        '#cache' => [
          'contexts' => [
            'url.site',
          ],
        ],
      ];
    }
    return $elements;
  }

  /**
   * Return a list of extension supported by Viewerjs's PluginLoader.
   */
  protected function viewerjsSupportedExtensions() {
    return array(
      'pdf',
      'odt',
      'odp',
      'ods',
      'fodt',
      'jpg',
      'jpeg',
      'png',
      'gif',
    );
  }
}