The bugfix was put in after the 3.2 beta was built. It should be in the 3.2 GA release:
Date: Sat, 15 Jul 2006 06:56:48 -0700 (PDT)
From: Demo User One <user1@example.com>
To: user2 <user2@example.com>
Message-ID: <10617394.01152971808765.JavaMail.SYSTEM@EXAMPLE >
Subject: testing attachments
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="----=_Part_0_18999844.1152971808671"
------=_Part_0_18999844.1152971808671
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Testing!
------=_Part_0_18999844.1152971808671
Content-Type: image/gif
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="=?utf-8?Q?=E8=AF=AD=E8=A8=80=E5=B7=A5=E5=85=B7.gif?="
R0lGODlhEAAQAJEAAP////8AmWaZ/wBmzCH5BAAAAAAALAAAAAAQABAAAAImhI+pyzjTjngwgins
fSJH+miYtnnWKA5mhJJMAMByTM/1Pcs5fhcAOw==
------=_Part_0_18999844.1152971808671--