Create a topic and add posts and add an attachment at some point in the topic. Then split posts from the topic including the attachment post into a new topic. The original topic still shows the attachment icon on viewforum.php as seen here,

http://www.jackal-tech.co.uk/phpBB3/viewforum.php?f=2" target="_blank

The "Welcome to phpBB 3" topic no longer has an attachment in it.