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

1. 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).

2. In most cases, you will also need to remove the same sub key from HKEY_CLASSES_ROOT as well.

Even easier method that I found, it just requires the command line.

1. Open a command prompt with Administrator privileges (right click > Run as Administrator).
2. Run the command assoc .<file_extension> in the prompt. This will give the file type which is associated with the extension. If this command returns File association not found for extension .<file_extension>, it means the extension is not associated with any file type. If it returns an association value in format .<file_extension>=<Some Value> , then go to Step 3.
3. Run the command ftype [FileType]= , where FileType is the file type returned in Step 2.

Special thanks go to My Digital Life for the instructions that I used to figure this one out. They also included a step that disassociates the extension from the file type, but based on what you are looking for, that isn't strictly necessary.

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.