Hi,
Indeed, to add an attribute to the session, you need to use the set method.
But the problem here, is that you are just declaring a first scenario in the middle of nowhere, and you don't keep a reference to it. Then you are declaring the real scenario that you are going to use. If you want something to happen in your scenario, that's in this scenario that you should declare things. Gatling will not magically link your two scenarios.
Furthermore, ElFileBody is a method to generate a body from a file. Its argument is a path, not a body. What you need to use is the StringBody method.
And why put this variable in the session ?