Let's take a look at each. You can name it whatever you want without effecting the stationery. You can store it anywhere. I would suggest putting it somewhere close to your desktop for easy access. You will not run into conflict with other stationeries with the same file names.
Some people just have a problem using the normal format.
What works on one computer, sometimes does not work on another. This new .eml format seems to work for everyone. If you get tired of a stationery and want to delete it there isn't any guess work if you have gotten all the files it came with. The preview pane is where you normally get to browse through your stationery folder and look at the pictures for each stationery. Then type TEXT GOES HERE in your letter.