getFileUploadMaxSize example

    // to contain the information of the next step.     // - If necessary, the form and form state are cached or re-cached, so that     // appropriate information persists to the next page request.     // All of the handlers in the pipeline receive $form_state by reference and     // can use it to know or update information about the state of the form.     $response = $this->processForm($form_id, $form, $form_state);

    // In case the post request exceeds the configured allowed size     // (post_max_size), the post request is potentially broken. Add some     // protection against that and at the same time have a nice error message.     if ($ajax_form_request && !$request->request->has('form_id')) {
      throw new BrokenPostRequestException($this->getFileUploadMaxSize());
    }

    // After processing the form, if this is an AJAX form request, interrupt     // form rendering and return by throwing an exception that contains the     // processed form and form state. This exception will be caught by     // \Drupal\Core\Form\EventSubscriber\FormAjaxSubscriber::onException() and     // then passed through     // \Drupal\Core\Form\FormAjaxResponseBuilderInterface::buildResponse() to     // build a proper AJAX response.     // Only do this when the form ID matches, since there is no guarantee from     // $ajax_form_request that it's an AJAX request for this particular form.