AHAH enhanced comment_upload_js works for 1 attachment

After the d.o. upgrade session Merlinofchaos noticed that AHAH behaviour doesn't reattach to the "attach" button. This is because form_clean_id sees edit-attach coming by twice (due to all the rebuilds going on). Inserting a form_clean_id(NULL, TRUE) on strategic places, or setting '#id' => 'edit-attach' on $form['new']['attach'] in comment_upload_upload_form brings forth a rather nasty bug:

1 - upload file 1 via the attach button
- result: file 1 is displayed in the attachment table.
2 - upload file 2 via the attach button
- result file 1 is "replaced" in the attachment table.

Multiple attachments work on d.o. atm because AHAH.js doesn't reattach behaviour (due to the id change) forcing a full form submit, which correctly supports multiple attachments.