So I will admit I am a bit new to XML, but here goes.
So it appears your problem is with this line in your request, the ASM thinks that xop:Include is an element, and since xop:Included is not a defined element in your schema attached to the XML Profile that is why you are getting blocked.
<sch:inhalt><xop:Include href="cid:file.pdf"/>sch:inhalt>
When I look at the information on
http://www.w3.org/TR/SOAP-attachments I can see why it thinks that is an element as it matchs the format I am seeing in examples here. Based just upon this it would appear the correct format for this line should be.
<inhalt href="cid:file.pdf"/>
However considering what you are wanting to do with base64 encoding I am not sure that is entirely correct.
Lastly from an ASM policy perspective you may want to make sure that Allow SOAP Attachments is turned on in your XML Profile.
Hope this helps a little and I am not telling you what you already know.
Mike