Interesting. I use outlook, and the body is:

<meta http-equiv="Content-Type" content="text/html; charset=utf-8"><h2
+>THIS IS HTML</h2>
[download]

And the headers are:

MIME-Version: 1.0
Subject: Test Message
Content-Type: multipart/alternative;
    boundary="----------=_1455830463-9817-0"
X-Mailer: MIME-tools 5.414 (Entity 5.414)
[download]

Could be a problem with mail::mailer, not sure at this point.