class FilemimeFormatter

class FilemimeFormatter extends BaseFieldFileFormatterBase {
  
  /**
   * {@inheritdoc}
   */
  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    return parent::isApplicable($field_definition) && $field_definition->getName() === 'filemime';
  }
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    $settings = parent::defaultSettings();
    $settings['filemime_image'] = FALSE;
    return $settings;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $form = parent::settingsForm($form, $form_state);
    $form['filemime_image'] = [
      '#title' => $this->t('Display an icon'),
      '#description' => $this->t('The icon is representing the file type, instead of the MIME text (such as "image/jpeg")'),
      '#type' => 'checkbox',
      '#default_value' => $this->getSetting('filemime_image'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function viewValue(FieldItemInterface $item) {
    $value = $item->value;
    if ($this->getSetting('filemime_image') && $value) {
      $file_icon = [
        '#theme' => 'image__file_icon',
        '#file' => $item->getEntity(),
      ];
      return $file_icon;
    }
    return $value;
  }

}