Can't change Windows 10 default file type association [duplicate]

Unfortunately, this is a known issue caused by a recent Windows update, and it is tentatively scheduled to be fixed later this month:

Symptom
After installing this update, some users cannot set Win32 program defaults for certain app and file type combinations using the Open with… command or Settings > Apps > Default apps.

In some cases, Microsoft Notepad or other Win32 programs cannot be set as the default.

Workaround
In some cases, attempting to set application defaults again will succeed.

Microsoft is working on a resolution and estimates a solution will be available in late November 2018.

Sources:
October 9, 2018—KB4462919 (OS Build 17134.345)
Microsoft Broke Windows 10’s File Associations With a Botched Update

Update

This problem should be resolved with the release of the KB4467682 Windows 10 Quality Update, which will increment the operating system build to 17134.441:

Addresses an issue that prevents some users from setting Win32 program defaults for certain app and file type combinations using the Open with… command or Settings > Apps > Default apps.

Sources:
November 27, 2018—KB4467682 (OS Build 17134.441)
Microsoft Fixes Windows 10’s File Association Bug, Unless You’re Using the October Update