How to add an attachment to a response using groovy script

I'm using SoapUI to produce some mock versions of external services to test an application in development.

One of these services is a cloud based data store, and so I'm saving the attachments from incoming requests to a local folder, and want to return them again when the appropriate get request is made.

It's easy enough to save the attachments coming in to a response, but I can't work out how to add an attachment to a response using the response groovy script. Is there a way to dynamically add attachments to responses or do I need to find a work around?

Thanks