A file is named:
XY.doc
AB.doc
IO.mp3
Having the last three character means that you get the type of the file. You can decide whether a file is a mp3 file or whether it isn't. At least that's true with standard file endings. However there's no gurantee that the ending is 3 characters long.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.