class FileExtensionFormatter

class FileExtensionFormatter extends BaseFieldFileFormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        $settings = parent::defaultSettings();
        $settings['extension_detect_tar'] = FALSE;
        return $settings;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $form = parent::settingsForm($form, $form_state);
        $form['extension_detect_tar'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Include tar in extension'),
            '#description' => $this->t("If the part of the filename just before the extension is '.tar', include this in the extension output."),
            '#default_value' => $this->getSetting('extension_detect_tar'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function viewValue(FieldItemInterface $item) {
        $filename = $item->value;
        if (!$this->getSetting('extension_detect_tar')) {
            return pathinfo($filename, PATHINFO_EXTENSION);
        }
        else {
            $file_parts = explode('.', basename($filename));
            if (count($file_parts) > 1) {
                $extension = array_pop($file_parts);
                $last_part_in_name = array_pop($file_parts);
                if ($last_part_in_name === 'tar') {
                    $extension = 'tar.' . $extension;
                }
                return $extension;
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public static function isApplicable(FieldDefinitionInterface $field_definition) {
        // Just show this file extension formatter on the filename field.
        return parent::isApplicable($field_definition) && $field_definition->getName() === 'filename';
    }

}