Mandatory custom fields, on edit, always require a new upload (regression)

This seems to be a regression of the bug described in CRM-1620 :

• create a mandatory custom field on a contact record
• create a new contact, upload a file in the custom field
• edit contact, click save: CiviCRM will output an error that the file is mandatory. (but it has already been uploaded)

The fix from CRM-1620 seems to be to check if there is already a value in Core/BAO/CustomGroup.php

      $groupId = CRM_Utils_Array::value('id', $group);
      foreach ($group['fields'] as $field) {
        $required = CRM_Utils_Array::value('is_required', $field);
        //fix for CRM-1620
        if ($field['data_type'] == 'File') {
          if (isset($field['customValue']['data'])) {
            $required = 0;
          }
        }

However, inspecting $field in 4.3 doesn't show any $field['customValue']['data'].