I had a problem recently that the file icons in Windows Explorer for all the filetypes set to open in Notepad++ were coming up as the blank icon. To fix the issue, I had to reset the icon handler for Notepad++. I’m not sure if this was related to an update for Notepad++, or something I did. Anyhow, the solution was to delete the registry key
then open Notepad++ with administrator rights, remove/add a file association, and restart Explorer. Problem fixed!