hello @Mark-Garvey
base on your question, I give solution is below
Step 1:- Go to the Window Setting.
Step 2:- Change default Program by extension.(.txt)
Step 3:- Switch from notepad++ into notepad.
Step 4:- Back to Notepad++ again.
Step 5:- Refreshing Icon Image.
I hope above solution is useful to you.
Thank you…