[Andreas]

Nice Hack. But you might want to consinder releasing it as a Plugin to avoid File Edits:

Hook: forumdisplay_query

PHP Code:

if (in_array($forumid, array(XX, YY))
{
    $hook_query_fields .= ", attachment.attachmendid AS attachmentid";
    $hook_query_joins .= " LEFT JOIN " . TABLE_PREFIX . "attachment as attachment ON (attachment.postid = thread.firstpostid)";
} 


To make it even more Userfriendly, you could add a setting Show Attachment Thumbnails to Forum Manager, so Users could easily turn this On/Off for individual Forums.