I posted about this a while ago, but I have ported _ck_’s bb_attachments plugin to BP 1.1.x. I took some time today to clean up the readme files and submit the plugin to the wordpress.com plugin database. Nothing about the plugin itself, or the hooks you’ll have to put into your theme, has changed since my last post; but at least now you’ll receive the file with full documentation and can use the WP plugin installer to install and maintain this bad boy.
Get it here or by searching for “forum attachments” on WP’s Add New Plugin page.
Bug reports (and especially patches!) are welcome.