Overwriting header in wiremailsmtp

[pout]

E-Mail-Client doesn't show a linked Image, so I want to try another content type header, but I don't get it overwritten.
I tried:

$headers =  array("Content-Type" => "multipart/related; boundary=\"abcdef\"; type=\"application/xhtml+xml\"");
if ($mail->mailHTML($to,$subject, $body, $headers)) { ... }

(not sure about the type=\"application/xhtml+xml\" part)
, but content type header still is:

Content-Type: multipart/alternative; boundary="f73ea6d91e70b02e6c46d1cd4b36a0f1"

or any other suggestions why

<img src="https://'.$config->httpHost.$img->url.'" height="180" width="180">

in body is not loading image in e-mail client (Thunderbird and also Android K-9 Mail). In Webmail this is working.
I do this in an external script for a cronjob.

[pout]

There is the answer.