The FMGo Developer Guide states: Saving and sending records as Excel, Snapshot Link is not supported.
Then the following link states that saving snapshot links are supported.
Try saving the snapshot to the document path to verify that the snapshot is being created.
I don't have a ready answer, but I would try including $snapshot and also capturing and sending get ( lasterror ) after the snapshot save step. That should get you some of the clues you would normally get via debugger, and might at least let you know whether the issue is with the fmpsl or with the attaching.