Still a problem, but I just discovered I wasn't setting the attachments.storageType to UCM. I also now set the attachment.revision to the revision number for the document seen in UCM. Not sure it matters, but I set the attachment.attachmentScope to TASK, and I set the name and title too.
But now I get this error on the User Activity when I create a test composite:
<bpelFault><faultType>1</faultType><operationErroredFault xmlns="http://xmlns.oracle.com/bpel/workflow/taskService"><part name="payload"><operationErroredFault xmlns="http://xmlns.oracle.com/bpel/workflow/taskService"><faultInfo>oracle.stellent.ridc.protocol.ServiceException: Unable to execute service DOC_INFO and function QdocInfo. The error was caused by an internally generated issue. The error has been logged.</faultInfo></operationErroredFault></part></operationErroredFault></bpelFault>
Clearly there is something more I must set, but I don't know what. Any and all help would be appreciated.