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),
        ]));
    }

}