public static function process(array $element, FormStateInterface $form_state) {
    $element['fid'] = [
      '#type' => 'hidden',
    ];
    $element['upload'] = $element['remove'] = [
      '#type' => 'submit',
      '#is_button' => TRUE,
      '#limit_validation_errors' => [
        $element['#parents'],
      ],
      '#weight' => 100,
    ];

    $element['upload']['#value'] = t('Upload');
    $element['upload']['#submit'][] = [static::class, 'upload'];

    $element['remove']['#value'] = t('Remove');
    $element['remove']['#submit'][] = [static::class, 'remove'];

    $key = array_merge($element['#parents'], ['fid']);
    // Don't use $form_state->hasValue(), because it will return TRUE if the
    // value exists and is falsy. Valid file IDs will always be truthy.
    $fid = $form_state->getValue($key);

    if ($fid) {
      $element['fid']['#value'] = $fid;

      $element['file'] = [
        '#theme' => 'file_link',
        '#file' => File::load($fid),
      ];
      $element['upload']['#access'] = FALSE;
    }
    else {
      $element['file'] = [
        '#type' => 'upload',
        '#title' => $element['#title'],
        '#upload_location' => $element['#upload_location'],
        '#upload_validators' => $element['#upload_validators'],
      ];
      $element['remove']['#access'] = FALSE;
    }
    return $element;
  }