ConfigImportForm::buildForm

public ConfigImportForm::buildForm(array $form, FormStateInterface $form_state)

public function buildForm(array $form, FormStateInterface $form_state) {
  $directory = config_get_config_directory(CONFIG_SYNC_DIRECTORY);
  $directory_is_writable = is_writable($directory);
  if (!$directory_is_writable) {
    drupal_set_message($this->t('The directory %directory is not writable.', ['%directory' => $directory]), 'error');
  }
  $form['import_tarball'] = array(
    '#type' => 'file',
    '#title' => $this->t('Configuration archive'),
    '#description' => $this->t('Allowed types: @extensions.', array('@extensions' => 'tar.gz tgz tar.bz2')),
  );

  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => $this->t('Upload'),
    '#disabled' => !$directory_is_writable,
  );
  return $form;
}