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

"Find the file extension in question under this key in the registry:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts

Deleting the sub-key with the same name as the extension you want to un-associate will delete the default program association. In older Windows versions, you'll have to kill and restart explorer.exe for this to take effect (the change is instant as of Windows 10 64-bit).
In most cases, you will also need to remove the same sub key from HKEY_CLASSES_ROOT as well."

"In Explorer, go to Tools | Folder Options | File Types, find the extension you're looking for, and click Delete.

Or, if Delete is grayed out, click Restore (to set it back to the defaults), then Delete."

I could not find the correct Windows Registry line, so I used a work around. On my desktop, I created a text file named "deleteable.txt". I changed the extension to .exe. Next, I used the control panel to change the default program assigned to the desired file extension to the new "deleteable.exe". I then closed the control panel, and finally deleted the desktop file.
I could not find the correct Windows Registry line, so I used a work around. On my desktop, I created a text file named "deleteable.txt". I changed the extension to .exe. Next, I used the control panel to change the default program assigned to the desired file extension to the new "deleteable.exe". I then closed the control panel, and finally deleted the desktop file.

"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):

Create a new empty file called something like foo.exe.
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.
Delete foo.exe.
Double click the file again and dismiss the Open with box (this might not be necessary).
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

Log off and in again."

"Creative Element Power Tools is a collection of productivity tools for Microsoft Windows Vista, Windows XP, Windows Server 2003, Windows 2000, and Windows Me. One of the file utility is File Type Doctor which allows user right-click any file and select Edit File Type to modify the applications used to open files of that type. Use File Type Doctor to choose default programs, add or remove file types from Explorer’s New menu, and customize files’ context menus.

More utilities here: Windows Vista File Associations Advanced Editing Management Tools"