Remove file associations on Windows 8

Create a blank file with a random file extension. Let's say <filename>.blaw.

 1. Open Notepad
 2. Menu *File* → *Save As*
 3. Give a name with your extension.
 4. And save it on your desktop (you can also save it at any other
    location).

Now right click your file (*.xlsx in this case) and select Open with → Choose Default Program → scroll down and select Browse for an App on this PC.

Now browse to filename.blaw and select it.

When your *.xlsx file is associated with <filename>.blaw, all you need to do is to delete the <filename>.blaw file. Now it'll force any *.xlsx file to make a file association again, and you can choose application from which you want to open your file, or leave it as it is.

And this solution certainly works!

The same process also works for Windows 10.

I think you missed the other SystemFileAssociations xlsx list in HKCR.

Windows Registry Editor Version 5.00

[-HKEY_CLASSES_ROOTS\SystemFileAssociations\.xlsx]
[-HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.xlsx]

To be sure everything is good, next go here and check the default setting on xlsx, usually it will point to another key in HKCR or sometimes have a long GUID referencing a persistent handler.

HKEY_CLASSES_ROOT\.xlsx

Either way we need to then jump to HKCR\xslx_auto_file or search for the GUID of the persistent handler and make sure everything looks chrome free.

FileTypesMan is free utility that you can use to change associations or even better yet (the way I used it):

I opened FileTypesMan.exe, selected the extension with the wrong application association, and clicked on menu Edit → Open File type in Regedit... Bam, it went right to it... Then I went to the open key under the filetype, cleared the data in the default regkey, closed regedit, and refreshed my Windows Explorer (F5). Wallah, done!