Creating a .mod file

I've recently acquired a macbook pro and would like to create a mod for some of the games I play (via the Steam platform, if that helps). This requires me to write a .txt file and then convert that file into a .mod file, which the game would then read an execute. However, while I am able to create a .txt file via TextEdit I can't find a way to convert it to a .mod file. I've tried a workaround for this - creating the file on my old windows computer, then emailing it across...but when downloading it from my email the macbook automatically converts the file into a .mpv file (which of course it isn't, and so is no longer usable). And suggestions for overcoming either issue gratefully received!

Thanks.

E-W.