Unable to upload files on Pantheon

When i try to upload files on a Pantheon Environment i get the following error message:

The specified file temporary:// could not be copied, because no file by that name exists. Please check that you supplied the correct filename

because it's a multi server setup there is no fixed location for a temporary file.

Pantheons documentation mentions adding the following line of code however i couldn't get it to work:

<?php
if (isset($_ENV['PANTHEON_ENVIRONMENT'])) {
  $conf['plupload_temporary_uri'] ='private://tmp';
}
?>