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;
    }

}