Ahh, thank you very much @jbudd. @knolleary, could you please add that information to the help section? I'd have had no idea otherwise, though I did look in the help item.
I know that I'm a NR noob, but what I find confusing is that I feel like the terminology changes in different parts of the interface. Sometimes there are references to msg.xyz, other times just as xyz
Perhaps it's just that I've missed the right-aligned text because I didn't know that it implied that it meant msg.xyz. With that new knowledge, I'll keep an eye out for specific examples of that. (Maybe I will find none )
In reply to this specific instance, I feel it's impossible to know if filename is just the name of a file (static) or some kind of variable. I'm pretty sure that I've seen other textbox hints that said things like msg.filename.