Gmail displaying plain text instead of HTML - php

Question

Situation
I have a script that is downloading emails from a Gmail account, tweaking the content, and re-sending out the emails.
Problem
Whenever I create an email with an embedded image, Gmail displays the plain text version of the email and the embedded image appears like a regular attachment.
Tested
If I use a different client, the image displays properly. If I forward the email from Gmail to a different client, the image displays properly. Multi-part emails that have regular attachments, or that have both plain text and html parts display properly - as long as they don't have an embedded image.
Email format
From what I can see, the format of the email is correct, but I've pasted it below, trimmed for brevity & privacy. The 2 things I notice is that the boundary appears to be properly set, and the img src cid: matches the Content-ID of the image.
MIME-Version: 1.0
Content-Type: multipart/related;
boundary="_=_swift_1575407655_756d311b6808cc9e07d75ccca0ed9b6c_=_"
--_=_swift_1575407655_756d311b6808cc9e07d75ccca0ed9b6c_=_
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
This email has been sent to a Mailing List. You can approve or reject it a=
t [URL here]
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
[image: noice.gif]
-----------------------
[Signature here]
--_=_swift_1575407655_756d311b6808cc9e07d75ccca0ed9b6c_=_
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable
<p>
This email has been sent to a Mailing List. You can approve or reject =
it at [URL here]</p>
<hr />
<div dir=3D"ltr"><div><img src=3D"cid:7f82a31e4f084e8f0a25edd913ed3aa2#swif=
t.generated" alt=3D"noice.gif" width=3D"474" height=3D"244"><br></div><div>=
<div dir=3D"ltr" class=3D"gmail_signature" data-smartmail=3D"gmail_signatur=
e"><div dir=3D"ltr"><div><div dir=3D"ltr"><div><div dir=3D"ltr"><br><br>---=
--------------------<br>[signature here]<br></div>=
</div></div></div></div></div></div></div>
--_=_swift_1575407655_756d311b6808cc9e07d75ccca0ed9b6c_=_
Content-Type: image/gif; name=noice.gif
Content-Transfer-Encoding: base64
Content-ID: <7f82a31e4f084e8f0a25edd913ed3aa2#swift.generated>
Content-Disposition: inline; filename=noice.gif
R0lGODlh2gH0APcAAAgDBRGJG5ZKGsDGB8lNFJJsGkkIBs+nLgnCM4lPZODnDqdMGo4rCVGIKs5q
[... bunch of base64 encoded stuff]
BFGgAUsAEvjkAlQgq/drOjfkFKdEETNBxl3bpi5lBYEAADs=
--_=_swift_1575407655_756d311b6808cc9e07d75ccca0ed9b6c_=_--
PHP code
Here is the PHP code that generates the Mailer object - which is a wrapper around Swiftmailer, and is used to do the actual sending:
$Mailer = new Mailer($subject_prepend.$this->subject);
// Simply a wrapper. Calls $this->Message->setBody($body,'text/plain');,
// where $this->Message is an instance of Swift_Message
$Mailer->setBody($plaintext_prepend.$this->plaintext);
$html_body = $html_prepend.$this->html;
if(count($this->Attachments)){
foreach($this->Attachments as $Attachment){
if($Attachment->isEmbedded()){
$image = \Swift_Image::fromPath($Attachment->getPath())->setDisposition('inline');
$cid = $Mailer->Message->embed($image);
//$cid = $Mailer->embedPath($Attachment->getPath());
$html_body = str_replace('cid:'.$Attachment->cid, $cid, $html_body);
}
else{
// calls $this->Message->attach(\Swift_Attachment::fromPath($path));
$Mailer->addAttachmentPath($Attachment->getPath());
}
}
}
// calls $this->Message->addPart($body,'text/html');
$Mailer->addHTMLBody($html_body);
The commented out line is how I was originally embedding the image, but it had the same result.
-
Is this some Gmail magic that's failing me, or is the email misconfigured somehow?
Edit
I managed to get the email sending properly with PHPMailer. The only difference I can see is that PHPMailer starts with:
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="b1_6hukH7nTGJu6fpr5tpXob5uQE7wXivW0oMppPNbwOi4"
Content-Transfer-Encoding: 8bit
where Swiftmailer starts with
MIME-Version: 1.0
Content-Type: multipart/related; boundary="_=_swift_1575416351_99d22ee774049152f712bc5ae65340fb_=_"
ie: PHPMailer uses multipart/alternative rather than multipart/related, and sets Content-Transfer-Encoding.
Also, all textual parts of SwiftMailer's email was `Content-Transfer-Encoding: quoted-printable' whereas PHPMailer didn't set that header.