http://www.phpbb.com/community/viewtopic.php?f=46&t=1700065

1) Create a post/topic and put 2 or more attachments
2) Once you post it, go in edit mode
3) Erase one of the attachment
4) Press CANCEL (This is important, as if you press "submit" there is no problem)
5) ... all the attachments have disappeared..

I checked the DB, searching for a fix and I noticed that post_attachment in the table phpbb_posts is 0.. I do not know what this is supposed to be.... but.. when I put it back to 1 the attachments appear again.