The cover art uploader should work in Safari 6+, http://caniuse.com/#feat=fileapi and http://caniuse.com/#feat=filereader say they're supported and http://html5demos.com/file-api apparently works fine. However people using Safari are still getting the old uploader.

IRC log: http://chatlogs.musicbrainz.org/musicbrainz/2013/2013-11/2013-11-05.html#T18-48-08-551935