I can't speak to having SmarterMail convert the file name for you, but you can associated .AFC extensions to automatically use NOTEPAD to open the file.
Just download a file to your desktop, or another location, and then right click and select OPEN WITH. Then select NOTEPAD and, before opening the file, click on the box, at the bottom left of the window, that allows you to "always use this program to open the file."
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