revert first part of change in #65 again, fixes #168

exercises/Exercise.php

@@ -115,7 +115,7 @@ abstract class Exercise extends SimpleORMap
             $this->options['comment'] = 1;
         }
 
-        if (!$request['files_visible']) {
+        if ($request['file_ids'] && !$request['files_visible']) {
             $this->options['files_hidden'] = 1;
         }
     }