what about this, then:

Mime-Version: 1.0

plain text
<x-html>

Mime-Version: 1.0

<x-html>

Mime-Version: 1.0

<x-html>


and result should be:

Mime-Version: 1.0

plain text
<x-html>

Mime-Version: 1.0
Content-Type: text/html

<x-html>

Mime-Version: 1.0
Content-Type: text/html

<x-html>