After some research I found that the wsdl defines what the expected content-type of the MTOM attachment is:
<xs:element minOccurs="0" name="inhoud" type="xs:base64Binary" xmime:expectedContentTypes="application/pdf"/>
I've filed a support ticket 48699
xs:base64Binary doesn't have a contentType attribute.
What happens when you change it to
<xs:element minOccurs="0" name="inhoud" type="xmime:base64Binary" xmime:expectedContentTypes="application/pdf"/>