Improve user interface for mass attachment downloader (all posts, all topics)

The user interface for downloading all posts or all topics needs further improvements. Right now it just doesn't look right (see screenshot). Maybe the download all features could be hidden behind some dropdown somewhere. Could even use JavaScript because this is a rather optional feature.

See: http://area51.phpbb.com/phpBB/viewtopic.php?p=244609#p244609