With file extensions, it helps to provide information where you got them from(say, they're from an 80s software which does not exist anymore, found them in Downloads/ThisIsANastyVirus.zip etc.).
If you have received the file by email, ask the sender or if you don't know the sender, just scrap it. It's probably not worth your time and potentially harmful.
And then, yes, trying notepad to open them might give you some insight, since often files have at least some portions of text. If notepad complains about encoding or it not being a textfile, download a hex editor like this onehttp://mh-nexus.de/en/hxd/
and open the file with it.
If you can't make anything of what it shows, post a screenshot here and I'll have a look at it. Dunno if I can figure out what it is, but I can at least try.
Or have a look here:http://en.wikipedia.org/wiki/FictionBook