Drupal 8, Add process function to managed_file field.

gistfile1.txt

    $default_process_fn = \Drupal::service('element_info')->getInfoProperty('managed_file', '#process', NULL);
    $form['field_photo'] = [
      '#type' => 'managed_file',
      '#required' => TRUE,
      '#title' => $this->getFieldTitle($product, 'field_photo'),
      '#description' => $this->getFieldDescription($product, 'field_photo'),
      '#upload_location' => 'public://',
      '#process' => array_merge($default_process_fn, [[get_class($this), 'managedFileProcess']]),
      '#upload_validators' => [
        'file_validate_extensions' => [
          $this->getAvailableExtensionsFromFieldSettings('field_photo', 'photo_print'),
        ],
      ],
    ];
    
  /**
   * Process managed file.
   */
  public static function managedFileProcess(&$element, FormStateInterface $form_state, &$form) {

    /**
     * After upload file, $form_state['values'] is clean. Code below prevent this problem.
     * Quite perhaps it's Drupal bag.
     *
     * Related issue:
     * https://www.drupal.org/node/2311199
     * https://drupal.stackexchange.com/questions/242641/drupal-8-theme-settings-form-api-remove-permanent-managed-file
     *
     * Problem place - drupal_validate_form().
     */
    $element['upload_button']['#limit_validation_errors'] = FALSE;
    $element['remove_button']['#limit_validation_errors'] = FALSE;

    return $element;
  }