Force Content-Transfer-Encoding=binary for XOP attachments

We had a problem similar as described in http://forum.soapui.org/viewtopic.php?t=2060 with one of our clients and Oracle Service Bus. For all text/* content types SoapUI sends encoded XOP parts instead of raw binary (in our case Content-Transfer-Encoding: quoted-printable)

Although the MTOM/XOP specification is not explicit on the value of Content-Transfer-Encoding, all interop examples always show binary mode (even for text/xml binaries) (http://www.w3.org/2000/xp/Group/4/08/implementation.html) 

The major goal of MTOM/XOP is to avoid encoding binaries and send them in raw format, so using other encoding formats like quoted-printable is strange.  This was confirmed by our vendor Oracle. They do support other Content-Transfer-Encoding for SOAP /w Attachment, but not for MTOM.

Can you please review the behavior in SoapUI? I did notice that when text/xml files are imported as base64 (insert file as base64) SoapUI already uses Content-Transfer-Encoding: binary  even when mime:contentType="text/xml". This pull request makes the behavior identical to the binaries in the attachment tab and only applies to MTOM. Thanks!