class File

class File extends FormElement {
  
  /**
   * {@inheritdoc}
   */
  public function getInfo() {
    $class = static::class;
    return [
      '#input' => TRUE,
      '#multiple' => FALSE,
      '#process' => [
        [
          $class,
          'processFile',
        ],
      ],
      '#size' => 60,
      '#pre_render' => [
        [
          $class,
          'preRenderFile',
        ],
      ],
      '#theme' => 'input__file',
      '#theme_wrappers' => [
        'form_element',
      ],
      '#value_callback' => [
        [
          $class,
          'valueCallback',
        ],
      ],
    ];
  }
  
  /**
   * Processes a file upload element, make use of #multiple if present.
   */
  public static function processFile(&$element, FormStateInterface $form_state, &$complete_form) {
    if ($element['#multiple']) {
      $element['#attributes']['multiple'] = 'multiple';
      $element['#name'] .= '[]';
    }
    return $element;
  }
  
  /**
   * Prepares a #type 'file' render element for input.html.twig.
   *
   * For assistance with handling the uploaded file correctly, see the API
   * provided by file.inc.
   *
   * @param array $element
   *   An associative array containing the properties of the element.
   *   Properties used: #title, #name, #size, #description, #required,
   *   #attributes.
   *
   * @return array
   *   The $element with prepared variables ready for input.html.twig.
   */
  public static function preRenderFile($element) {
    $element['#attributes']['type'] = 'file';
    Element::setAttributes($element, [
      'id',
      'name',
      'size',
    ]);
    static::setAttributes($element, [
      'js-form-file',
      'form-file',
    ]);
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
    if ($input === FALSE) {
      return NULL;
    }
    $parents = $element['#parents'];
    $element_name = array_shift($parents);
    $uploaded_files = \Drupal::request()->files
      ->get('files', []);
    $uploaded_file = $uploaded_files[$element_name] ?? NULL;
    if ($uploaded_file) {
      // Cast this to an array so that the structure is consistent regardless of
      // whether #value is set or not.
      return (array) $uploaded_file;
    }
    return NULL;
  }

}