I had a couple of topics that used external attachments for some images and found that they weren't being converted. Did a bit of spelunking through PhpbbToBbpress.php and found a typo in convertPosts() around line 816 ...
if ($wpdb->num_rows > 0)
I believe should be
if ($fdb->num_rows > 0)
Once I made that change my download links for the images showed up in the converted topic.