Hi guys,
I must somewhat disagree, allthough I also think that hidding the extension is bad, and I hate it, showing it does not appear to me as a big security advantage for average grandma.
Since .txt is good, .exe is bad, .ini is good, .reg is bad, .jpg is good unless you have your system unpatched, .avi is good, unless it downloads you some codecs, .rtf is bad since it actually may run word and be actually infected .doc, .ppt might be good, but actually is bad as well, .eml is good, but might contain attachments, .cmd is bad, .pif and .lnk might be good and bad at the same time, I am afraid I have already lost your attention and my grandma surely must be already sleeping....