mandrill打破了我的多部分哑剧消息

Question

Building off of PHPMailer attachments are showing up as base64-encoded text , I'm using PHPMailer to send emails using Mandrill.

Here's what the email looks like in my GMail inbox:

Here's what I want it to look like:

Here's the email that PHPMailer created:

Headers:

Date: Mon, 3 Sep 2018 17:23:30 -0500
From: [e-mail]
Message-ID: <l3dhUHTYkrQSd0hXkGUZDYNoAYKEkAIt21aBAknnpJE@aa1b216e11db>
X-Mailer: PHPMailer 6.0.5 (https://github.com/PHPMailer/PHPMailer)
MIME-Version: 1.0
Content-Type: multipart/mixed;
    boundary="b1_l3dhUHTYkrQSd0hXkGUZDYNoAYKEkAIt21aBAknnpJE"
Content-Transfer-Encoding: 8bit

Body:

This is a multi-part message in MIME format.
--b1_l3dhUHTYkrQSd0hXkGUZDYNoAYKEkAIt21aBAknnpJE
Content-Type: text/plain; charset=us-ascii

See attached!

--b1_l3dhUHTYkrQSd0hXkGUZDYNoAYKEkAIt21aBAknnpJE
Content-Type: application/csv; name="test.txt"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=test.txt

aGVsbG8sIHdvcmxkIQ==

Here's what I get in gmail:

Header:

From: Example <[e-mail]>
Sender: Example <[e-mail]>
Subject: Attachment Test 9184364
Return-Path: <[e-mail]>
Received: from [34.225.176.22] by mandrillapp.com id b19ad375f24f410ab92db42268b72992; Mon, 03 Sep 2018 22:23:32 +0000
Message-Id: <[e-mail]>
To: <[e-mail]>
X-Report-Abuse: Please forward a copy of this message, including all headers, to [e-mail]
X-Report-Abuse: You can also report abuse here: http://mandrillapp.com/contact/abuse?id=30776766.b19ad375f24f410ab92db42268b72992
X-Mandrill-User: md_30776766
Date: Mon, 03 Sep 2018 22:23:32 +0000
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="_av-oFI9mLO0ZP3IifF5xBDhHQ"

Body:

--_av-oFI9mLO0ZP3IifF5xBDhHQ
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit

This is a multi-part message in MIME format.
--b1_l3dhUHTYkrQSd0hXkGUZDYNoAYKEkAIt21aBAknnpJE Content-Type: text/plain;
charset=us-ascii See attached!
--b1_l3dhUHTYkrQSd0hXkGUZDYNoAYKEkAIt21aBAknnpJE Content-Type:
application/csv; name="test.txt" Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=test.txt aGVsbG8sIHdvcmxkIQ==
--b1_l3dhUHTYkrQSd0hXkGUZDYNoAYKEkAIt21aBAknnpJE--
--_av-oFI9mLO0ZP3IifF5xBDhHQ
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: 7bit

This is a multi-part message in MIME format.
--b1_l3dhUHTYkrQSd0hXkGUZDYNoAYKEkAIt21aBAknnpJE
Content-Type: text/plain; charset=us-ascii

See attached!

--b1_l3dhUHTYkrQSd0hXkGUZDYNoAYKEkAIt21aBAknnpJE
Content-Type: application/csv; name="test.txt"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=test.txt

aGVsbG8sIHdvcmxkIQ==

--b1_l3dhUHTYkrQSd0hXkGUZDYNoAYKEkAIt21aBAknnpJE--<img src="https://mandrillapp.com/track/open.php?u=30776766&id=b19ad375f24f410ab92db42268b72992" height="1" width="1">
--_av-oFI9mLO0ZP3IifF5xBDhHQ--

So it would appear that Mandrill is replacing the Multipart boundary and, in the process, is breaking my outgoing emails.

Any ideas as to what I can do to prevent Mandrill from breaking my outgoing emails?

Thanks!