Sorry, you are right. Execute from Browser is for execute Javascript as a string, not from a file.
I tried CallJS and got also an error when I call it in on start of Layout. It seems that CallJS is to fast in execute the Javascript. So I add a Wait of 0.1sec from System before CallJS in "on start of Layout" and now it works.
Maybe you have to alter the Wait-Time, but it seems that it works.