(I know that this thread is old but I just have lost a couple of hours in Google searching for a fix until I decided to fix it myself and I believe that other people can find it useful even today.)
The problem is in the "default" template. The fix is to edit the viewticket.tpl file in the default template folder. Change line 128 from this:
<img src="images/article.gif" align="middle" /> <a href="dl.php?type=ar&id={$reply.id}&i={$num}">{$attachment}</a><br />
to this:
<img src="images/article.gif" align="middle" /> {if $reply.admin}<a href="dl.php?type=ar&id={$reply.id}&i={$num}">{else}<a href="dl.php?type=a&id={$id}&i={$num}">{/if}{$attachment}</a><br />
Or just use another template your template .