Implements Drupal\Core\StreamWrapper\StreamWrapperInterface::getExternalUrl().
string Returns the HTML URI of a private file.
Overrides StreamWrapperInterface::getExternalUrl
function getExternalUrl() {
  $path = str_replace('\\', '/', $this
    ->getTarget());
  return url('system/files/' . $path, array(
    'absolute' => TRUE,
  ));
}