Similar to the top answer, but I find it best to only delete the UserChoice sub-key because you don't need to kill all of the info about the file extension (like the open with list), just the "choice the user selected" (aka-UserChoice). I use a REG file to make it fast to repeat for testing. You can copy paste this, including the version line, and save as a REG file. Just change .ics and .vcs to whatever extensions you want (or just remove one line if you only want to reset one):
Windows Registry Editor Version 5.00
[-HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.ics\UserChoice]
[-HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.vcs\UserChoice]
If you aren't familiar with REG files, the above lines that start with "-" simply delete those keys from the registry.
Also, and the main reason I posted this answer, with this solution I don't have to restart explorer.exe. Maybe something specific to my setup, but I've used on Win7 and Win8 and never had to restart explorer.exe for the change to take effect.
