Originally I got an unknown file type .abc;

Suppose I carelessly chose to open it with Real Player, ALWAYS.
And now I wanna undo that operation, letting .abc still be an unknown file type in my laptop. What shall I do?

Go to your folder options (for NT, it's under View-->Options),
select the File Types tab and remove the mapping bettwen file
type .abc and Real Player.

or if you want to change it to be open by other application, try
this way: select the file first, press the "Shift" then right-
click and chooose the "Open With" option, then select the application
you want from the application list.

Good luck!