ManagedFile::validateManagedFile

public static ManagedFile::validateManagedFile(&$element, FormStateInterface $form_state, &$complete_form)

public static function validateManagedFile(&$element, FormStateInterface $form_state, &$complete_form) {
  // If referencing an existing file, only allow if there are existing
  // references. This prevents unmanaged files from being deleted if this
  // item were to be deleted.
  $clicked_button = end($form_state->getTriggeringElement()['#parents']);
  if ($clicked_button != 'remove_button' && !empty($element['fids']['#value'])) {
    $fids = $element['fids']['#value'];
    foreach ($fids as $fid) {
      if ($file = File::load($fid)) {
        if ($file->isPermanent()) {
          $references = static::fileUsage()->listUsage($file);
          if (empty($references)) {
            // We expect the field name placeholder value to be wrapped in t()
            // here, so it won't be escaped again as it's already marked safe.
            $form_state->setError($element, t('The file used in the @name field may not be referenced.', ['@name' => $element['#title']]));
          }
        }
      }
      else {
        // We expect the field name placeholder value to be wrapped in t()
        // here, so it won't be escaped again as it's already marked safe.
        $form_state->setError($element, t('The file referenced by the @name field does not exist.', ['@name' => $element['#title']]));
      }
    }
  }

  // Check required property based on the FID.
  if ($element['#required'] && empty($element['fids']['#value']) && !in_array($clicked_button, ['upload_button', 'remove_button'])) {
    // We expect the field name placeholder value to be wrapped in t()
    // here, so it won't be escaped again as it's already marked safe.
    $form_state->setError($element, t('@name field is required.', ['@name' => $element['#title']]));
  }

  // Consolidate the array value of this field to array of FIDs.
  if (!$element['#extended']) {
    $form_state->setValueForElement($element, $element['fids']['#value']);
  }
}