You must edit some templates:
postbit_attachmentmoderated
Code:
<a href="attachment.php?$session[sessionurl]attachmentid=$attachment[attachmentid]" target="_blank">$attachment[filename]</a>
postbit_attachment
Code:
<a href="attachment.php?$session[sessionurl]attachmentid=$attachment[attachmentid]" target="_blank">$attachment[filename]</a>
postbit_attachmentthumbnail
Code:
<a href="attachment.php?$session[sessionurl]attachmentid=$attachment[attachmentid]" target="_blank"><img class="thumbnail" src="attachment.php?$session[sessionurl]attachmentid=$attachment[attachmentid]&stc=1&thumb=1" border="0" alt="<phrase 1="$attachment[filename]" 2="$attachment[counter]" 3="$attachment[filesize]">$vbphrase[image_larger_version_x_y_z]</phrase>" /></a>
The changes are marked bold.