In an activity open your attachment instance. To do that, first get the pzInsKey from an Obj-Browse on Data-WorkAttach-File class. Then use Obj-Open-By-Handle to open the whole instance. You will get a property named pyAttachStream which will contain the full byte stream for the file content.
Thank you for the reply, you have put me on the right track. Unfortunately I will not know any of the Data-WorkAttach-File properties to query with Obj-Browse. I only know the Category of the attachment which is not a property of Data-WorkAttach-File, but a property of Link-Attachment which then has a PageList of Data-WorkAttach-File. I can use Obj-Browse on the Link-Attachment and get to it that way but it would seem there should be a built in way to retrieve the Data-WorkAttach-File pages for a given Category of attachments?
For now I will use Obj-Browse way but would love to know if anyone knows a more OOTB way to retrieve the attachments for a given Category?