Actually they are all listed in BOL together as files, so i would agree with the Question. Helps I got it right as well 😉

ms-help://MS.SQLCC.v9/MS.SQLSVR.v9.en/udb9/html/39e00ebe-c9b7-4d53-90bd-15de2015fd34.htm

Not sure if that works for everyone, as I find it weird that my local BOL gives a web type address for the pages... So no idea if that's universal for all installs.

Actually I was in two minds about .ndf - I knew it was a suggested extension for secondary data files but wasn't sure if it was the default. Picked it anyway, so for once I got a fifty fifty right! I get a very high number of fifty fifty's wrong - which statistically speaking shouldn't the be the case 🙂