"Open With" Problem Windows 7

I've opened a dll file with notepad but I forgot to uncheck "always use the same program" section. After that it is started to open with that custom program (which is really normal) and I have changed the custom program and uninstalled that custom program. System won't recognize it now but how do I restore it to that warning while opening any dll file? "you are trying to open a dll file...". Shortly how to restore it back to its original form?