Drupal 8 - How to wrap a webform managed file field

example.module

<?php

/**
 * @file
 * Content of the example.module.
 * We start with a webform where a "product_complain" file managed field is present.
 * Our objective is to wrap this field in a <div> with a custom class.
 */

use Drupal\Core\Form\FormStateInterface;
use Drupal\example\RenderElementWrapper;

/**
 * Implements hook_webform_element_ELEMENT_TYPE_alter() for managed_file.
 */
function example_webform_element_managed_file_alter(array &$element, FormStateInterface $form_state, array $context) {
  // Add an extra theme wrapper to our attachment, for an easier styling.
  if ($element['#webform_id'] === 'product_complaint--attachment') {
    $element['#outer_wrapper'] = TRUE;
  }
}

/**
 * Implements hook_element_info_alter().
 */
function example_element_info_alter(array &$info) {
  \Drupal::classResolver()->getInstanceFromDefinition(RenderElementWrapper::class)
    ->alterElementInfo($info);
}

RenderElementWrapper.php

<?php

namespace Drupal\example;

use Drupal\Core\Form\FormStateInterface;

/**
 * Provides functionality to process render elements.
 */
class RenderElementWrapper {

  /**
   * Alters the element type info.
   *
   * Alter the 'process' information for the managed file elements.
   *
   * @param array $info
   *   An associative array with structure identical to that of the return value
   *   of \Drupal\Core\Render\ElementInfoManagerInterface::getInfo().
   */
  public function alterElementInfo(array &$info) {
    foreach ($info as $element_type => $element_info) {
      if ($element_type == 'managed_file') {
        $info[$element_type]['#process'][] = [static::class, 'processElement'];
      }
    }
  }

  /**
   * Process the managed_file elements.
   *
   * @param array $element
   *   An associative array containing the properties and children of the
   *   element. Note that $element must be taken by reference here, so processed
   *   child elements are taken over into $form_state.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   * @param array $complete_form
   *   The complete form structure.
   *
   * @return array
   *   The processed element.
   */
  public static function processElement(array &$element, FormStateInterface $form_state, array &$complete_form) {
    if (!empty($element['#outer_wrapper']) && !$form_state->isRebuilding()) {
      $element['#prefix'] = '<div class="managed-file-outer-wrapper">' . $element['#prefix'];
      $element['#suffix'] .= '</div>';
    }

    return $element;
  }

}