Just FYI, we use payloads to automatically make all the files in the temporary execution directory available to the users.  It's nice that a bit of code can be automatically run at the end of a code execution to send back information about what files were created.  Of course, this could be accomplished other ways (like a permanent comm channel open on which we query about files, or a post-execution hook of some type that will send this metadata on iopub.)