This question already has an answer here:

I have tried to open ex4 file by double clicking but could not open it and the dialog box pop up says to find appropriate program from the web. Meanwhile I tried to open the file with 'notepad' by mistakenly checking the 'Always open file of these type with' and afterwards found that all ex4 files when double clicking open up with notepad but does not show up the contents of the file. Now I want to revert back to the starting conditions when ex4 file on double clicking could not be open with any available editor with dialog box saying to find appropriate programs from the web,as I think it could be harmful to ex4 file without reverting back.Anyone with any knowledge.

marked as duplicate by Ƭᴇcʜιᴇ007, Tog, Karan, Gaff, Brad Patton Apr 13 '13 at 17:48

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Browse other questions tagged or ask your own question.