For quite some time, users have been reporting an inconsistent bug in Windows 10 that prevents them from changing the default program associated with a file type.
Windows 10 users are stating that when they try to change the default program used to open a particular file extension, such as .txt files, they are unable to associate it with popular Notepad replacements such as Notepad++. On the other hand, they have no problem changing the file association to a different one.
"This is a strange bug. It affects some applications, but not others," Chris Hoffman stated in an article on HowToGeek. "For example, Windows 10 won’t let you make Adobe Photoshop or Notepad++ a default application for images or text files.
But you can make other applications, like IrfanView, VLC, or Google Chrome, your defaults."
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