I expect <cfargument type="xml"> to work for SOAP and <cfargument type="binary"> to work for DIME. What I suppose you've got to find out is how to adapt your WSDL to handle DIME.
I don't think <cfargument type="binary"> will work with DIME.
I know this because I was building this webservice to be called by .NET code - and the .NET developer shows me the way he attaches an attachment using DIME. With DIME, the attachment is not one of the parameters of your webservice function.
Anyway we decided to go the non DIME way and just do it via the <cfargument type="binary"> which according to him is less efficient than using DIME.