It has been a while since I worked with ColdFusion. The last I remember, it did not provide a harness to send a SOAP attachment. I solved this issue by writing a custom CFX tag with Java that did it for me. The entire SOAP call will need to go through the tag.
The Java library you want to look at if you choose to do this is javax-ws. You also need to find out if the service call has to use MTOM.
Sorry that is not a direct solution, but it is what I had to do with CF a few versions back.