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;
    }

}