Although soapUI Pro supports MTOM, I've found that its support is fairly narrow and does not work in all circumstances. Currently, it appears that the schema must declare the element for the attachment as type="xmime:base64Binary".
If I select "Enable MTOM", then I would like for soapUI to use MTOM regardless of whether or not there is a xmime:base64Binary type present. Other SOAP testing tools are capable of this, as are many SOAP toolkits used for developing clients.
The MTOM specs do not seem to specify that xmime:base64Binary must be used. I've seen several implementations where the standard xsd:base64Binary is used. For example, the JAX-WS documentation states that xsd:base64Binary can be used [sup:297dutkk]1[/sup:297dutkk] as does the Apache Axis 2 documentation [sup:297dutkk]2[/sup:297dutkk].
My service uses xsd:base64Binary along with xmime:expectedContentType. So far, I am unable to successfully test my service using MTOM. Would you please consider improving MTOM support in soapUI? As mentioned earlier, I'd love to be able to force soapUI to use MTOM regardless of the schema definition.