The zip file created when exporting a forum discussion OR post stores attachments in a manner different from what the included HTML document expects. The structure in the zip file is /site_files/_/. but the HTML document just expects /site_files/. Steps to reproduce:

1. Create a forum.
2. Create a discussion.
3. Add a post with an attachment (doesn't matter what kind) to the discussion.
4. Export the discussion as a file download.

I've tried this with both an individual post and a full discussion with the same result. This is on my development system with the latest weekly release: Moodle 2.0.1+ (Build: 20110209).