In some situations when building the XML for MTOM attachments there will be a trailing semi-colon at the end of the Content-Type line in the header. This is causing some other software suites issues when trying to parse the data.
Content-Type: application/xop+xml; charset=UTF-8; type="text/xml";
There shouldn't be a semi-colon after the "type" value as it's the last of the Content-Type key/value pairs.
On line 129 of AttachmentSerializer you can see how the header gets built up and always adds a semi-colon.