Duplicate attachments using mimemail and smtp

My emails are being sent properly, but each attachment is sent twice. The root cause (bug perhaps) is that in smtp.mail.inc
$mailer->AddAttachment is called on line 432, and $mailer->AddStringAttachment is called on line 449 when mimemail is used. I solved the issue by modifying line 404 as such:

elseif ( strpos($body_part, 'Content-Disposition: attachment;') && !isset($message['params']['attachments']) ) {

since mimemail attachments are handled in the block on line 446.