Save an Argfile with what extension (if any) and what type of file under Windows?
I'm not sure I know what you mean by type, but you can save the argument file with any extension or no extension. Myself, I tend to use .arg or .args, but have also just used .txt at times.
Just as long as the actual contents of the argument file is plain text, I'm pretty sure exiftool will go ahead and read it.