class FileModuleTestForm

class FileModuleTestForm extends FormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'file_module_test_form';
  }
  
  /**
   * {@inheritdoc}
   *
   * @param array $form
   *   An associative array containing the structure of the form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   * @param bool $tree
   *   (optional) If the form should use #tree. Defaults to TRUE.
   * @param bool $extended
   *   (optional) If the form should use #extended. Defaults to TRUE.
   * @param bool $multiple
   *   (optional) If the form should use #multiple. Defaults to FALSE.
   * @param array $default_fids
   *   (optional) Any default file IDs to use.
   */
  public function buildForm(array $form, FormStateInterface $form_state, $tree = TRUE, $extended = TRUE, $multiple = FALSE, $default_fids = NULL) {
    $form['#tree'] = (bool) $tree;
    $form['nested']['file'] = [
      '#type' => 'managed_file',
      '#title' => $this->t('Managed <em>@type</em>', [
        '@type' => 'file & butter',
      ]),
      '#upload_location' => 'public://test',
      '#progress_message' => $this->t('Please wait...'),
      '#extended' => (bool) $extended,
      '#size' => 13,
      '#multiple' => (bool) $multiple,
    ];
    if ($default_fids) {
      $default_fids = explode(',', $default_fids);
      $form['nested']['file']['#default_value'] = $extended ? [
        'fids' => $default_fids,
      ] : $default_fids;
    }
    $form['textfield'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Type a value and ensure it stays'),
    ];
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Save'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    if ($form['#tree']) {
      $uploads = $form_state->getValue([
        'nested',
        'file',
      ]);
    }
    else {
      $uploads = $form_state->getValue('file');
    }
    if ($form['nested']['file']['#extended']) {
      $uploads = $uploads['fids'];
    }
    $fids = [];
    foreach ($uploads as $fid) {
      $fids[] = $fid;
    }
    \Drupal::messenger()->addStatus($this->t('The file ids are %fids.', [
      '%fids' => implode(',', $fids),
    ]));
  }

}