How do you set the filename for a JAXWS MTOM Attachment?

I'll update this with a code example in the morning but I have a basic JAX-WS implementation used to download a generated file via MTOM and the the file that get's attached when I test it in SOAPUI has a random filename even though the DataSource I'm passing the DataHandler has the correct filename via the getName() method on the DataSource implementation. Just trying to figure out what I'm missing.