Attaching files (mediaitems) and messages to a custom record

I've got to create a Message record and a File record, then attach them both to a custom record (on the Files and Messages sub lists)

I've been looking over the WSDL and am not seeing any promising fields.  I see transaction and activity on message, but don't see how I could use them with a custom record as I need to specify the custom record type id as well as instance id.  I see how i can add a file to a message, but not a file to a custom record.

If anyone has attached a message or file to a custom record and can point me in the right direction (or share a snippet of code) I would really appreciate it.