Attachments can only be done one at a time, and require going back and editing the artifact to add an additional attachment. This applies to Tickets and comments, at least.
Support for multiple=true would be sufficient.
The UI for adding attachments needs some cleanup as well, might as well do it right after adding support for multiple. On tickets, there is a checkbox that says "I would like to add an attachment" It's silly to have to click that little checkbox. The wiki has an "Add Attachment" button, which is better but probably too big (and buttons are better for big changes, not for just showing the
input type=upload field). I'd like to see a link that says "Add Attachments", just like is used for commenting. The positioning of the upload box could use some adjustment too. On some forms it shows up in a weird spot.