Attachment code should have a more centralized and reasonable path, which is used everywhere. Thumbnailing should be moved into that central place, and tracker, wiki and discussion should reuse more attachment related code.
Once that's done the comment attachment UI needs some work.
You ought to be able to add an attachement at the same time that you are editing the comment, not just after it's posted.
The add attachment flow should have the CSS tweeked so nothing gets clipped or cut off.
Attachments ought to be displayed as thumbnails, or as plain links and not added to the message in full size.
description has changed
Blocked on refactoring attachment code
description has changed