在Drupal中，如何验证使用webforms上传的文件的内容，如果验证失败，则*不*上传？

/**
 * Implements hook_form_alter().
 */
function mymodule_form_alter(&$form, &$form_state, $form_id) {
  // Check if the form is a webform.
  if (strpos($form_id, 'webform_client_form_') === 0) {
    // Add a custom validation function to the form.
    $form['#validate'][] = 'mymodule_webform_file_validation';
  }
}

/**
 * Custom validation function for webform file upload.
 */
function mymodule_webform_file_validation($form, &$form_state) {
  // Get the uploaded file from the form state.
  $file = file_save_upload('file_upload_element', array(
    'file_validate_extensions' => array('jpg', 'png', 'gif'), // Specify the allowed file extensions.
  ));

  // Check if the file validation failed.
  if (!$file) {
    form_set_error('file_upload_element', t('Invalid file format. Please upload a JPG, PNG, or GIF file.'));
  }
  else {
    // Perform additional validation if needed.
    // ...
  }
}