[TigerC10]

You can modify the template attachmentbit.

<!-- code corrected, see post #5 below -->
<if condition="[s]$vbulletin->userinfo['postcount'][/s] >= 50">
<tr>
	<td class="$bgclass">
		<img class="inlineimg" src="$stylevar[imgdir_attach]/$attachment[attachmentextension].gif" alt="<phrase 1="$attachment[attachmentextension]">$vbphrase[file_type_x]</phrase>" width="16" height="16" border="0" style="vertical-align:middle" />&nbsp;<a href="attachment.php?$session[sessionurl]attachmentid=$attachment[attachmentid]&amp;d=$attachment[dateline]" target="_blank">$attachment[filename]</a>
	</td>
	<td class="$bgclass">
		$attachment[filesize]
	</td>
	<td class="$bgclass" align="center">
		$attachment[counter]
	</td>
</tr>
<else />
<tr>
	<td class="$bgclass" colspan="3" align="center">You do not have enough posts to download attachments.</td>
</tr>
</if>

Change the 50 to whatever post count you want it to be. If you want to make it more advanced than that, you should post a mod request.