[1.0.X] Fixed #9367 - EmailMultiAlternatives does not properly handle attachments.

Thanks to Loek Engels for the bulk of the patch.

Backport of r10983 from trunk


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10984 bcc190cf-cafb-0310-a4f2-bffc1f526a37