File form element now has a value and can be required

The file element now has a value and can be set required. The value will be an array of \Symfony\Component\HttpFoundation\File\UploadedFile objects.

Previously, a form with a required file element, such as show below, would fail and could not be submitted.

 $form['file_test_upload'] = [
      '#type' => 'file',
      '#required' => TRUE,
    ];