HTTP Test Step MIME Base64

Hello everybody,

I am struggling with a test case and would appreciate your help. I have an HTTP service which accepts MIME multipart message - one part containing an XML document, the other a file encoded as BASE64. I am having problems testing this in SoapUI. In my current status I am able to create an HTTP request, select the message type as multipart/mixed,copy the XML, add the file as an attachment. When I hit send SoapUI correctly creates the MIME message, however the file is in binary format. Is there a way to get it encoded in BASE64?

Any help would be appreciated.

Kind regards,

Filip