The 'File' element is unavailable because a <a href="https://www.ostraining.com/blog/drupal/creating-drupal-8-private-file-system/">private files directory</a> has not been configured and public file uploads have not been enabled. For more information see: <a href="https://www.drupal.org/psa-2016-003">DRUPAL-PSA-2016-003</a>