This is aestetical. There should be a simple way to mark file upload mandatory with "red asterix" as it is in case of any other input.
I mean - I can set:
'allowEmpty' => FALSE, 'on' => 'insert'
but this does not enforce class="required" on the input label.
Sure - it's possible to workaround this through manual styling but... IMHO a "required" parameter should be added to file upload validator to faciliate this issue.