code: public function getImageFileExtensions(array $settings)
      {
          $languageService = $this->getLanguageService();
  
          $extensions = GeneralUtility::trimExplode(',', strtolower($GLOBALS['TYPO3_CONF_VARS']['GFX']['imagefile_ext']), true);
          // We don't consider PDF being an image...
          if ($key = array_search('pdf', $extensions)) {
              unset($extensions[$key]);
          }
          // ... neither SVG since vectorial
          if ($key = array_search('svg', $extensions)) {
              unset($extensions[$key]);
          }
          asort($extensions);
  
          $elements = [];
          foreach ($extensions as $extension) {
              $label = $languageService->sL('LLL:EXT:image_autoresize/Resources/Private/Language/locallang.xlf:extension.' . $extension);
              $label = $label ? $label : '.' . $extension;
              $elements[] = [$label, $extension];
          }
  
          $settings['items'] = array_merge($settings['items'], $elements);
      }
docs: Prepares a list of image file extensions supported by the
  current TYPO3 install.
  
  @param array $settings content element configuration
  @return void