Answer by binaryfunt for How do you remove a default program association for file types in Windows 7?

Safe brute-force method

None of the other answers' methods worked on their own for me when trying to delete the file association for something like .htaccess. This combined method worked (Windows 10):

1. Create a new empty file called something like foo.exe.

2. Go to a file with the extension you want to reset and Open with.... Make sure Always use this app to open .<extension> files is ticked. Then Look for another app on this PC and choose the foo.exe you just created.

3. Delete foo.exe.

4. Double click the file again and dismiss the Open with box (this might not be necessary).

5. Now open regedit and delete the following keys:

   HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.<extension>
   HKEY_CLASSES_ROOT\.<extension>
   HKEY_CLASSES_ROOT\Applications\foo.exe
   

6. Log off and in again.