Can you please specify what you mean by "envelope level" and "root part"? I tested the configuration and got the following result:
Without setting var://service/set-response-header/Content-Type
--5a5fe9c3-b910-4784-a321-b239e99a3ae7
Content-ID: <root>
Content-Type: application/xop+xml; type="application/soap+xml"
Content-Transfer-Encoding: binary
<?xml version="1.0" encoding="UTF-8"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:exmp="http://example.com">
<soapenv:Header/>
<soapenv:Body>
<exmp:element1>some text</exmp:element1>
<exmp:element2>some more text</exmp:element2>
<exmp:attachment><xop:Include href="cid:mytextpart" xmlns:xop="http://www.w3.org/2004/08/xop/include"/></exmp:attachment>
</soapenv:Body>
</soapenv:Envelope>
--5a5fe9c3-b910-4784-a321-b239e99a3ae7
Content-ID: <mytextpart>
Content-Transfer-Encoding: binary
Content-Type: text/plain
<binary content>
--5a5fe9c3-b910-4784-a321-b239e99a3ae7--
With var://service/set-response-header/Content-Type
--d867412b-08c3-4ef1-8bd4-b239e99abc8d
Content-ID: <root>
Content-Type: 'application/xop+xml; charset=utf-8; type="application/soap+xml"'
Content-Transfer-Encoding: binary
<?xml version="1.0" encoding="UTF-8"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:exmp="http://example.com">
<soapenv:Header/>
<soapenv:Body>
<exmp:element1>some text</exmp:element1>
<exmp:element2>some more text</exmp:element2>
<exmp:attachment><xop:Include href="cid:mytextpart" xmlns:xop="http://www.w3.org/2004/08/xop/include"/></exmp:attachment>
</soapenv:Body>
</soapenv:Envelope>
--d867412b-08c3-4ef1-8bd4-b239e99abc8d
Content-ID: <mytextpart>
Content-Transfer-Encoding: binary
Content-Type: text/plain
<binary content>
--d867412b-08c3-4ef1-8bd4-b239e99abc8d--
--HP
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