Default save extension in Notepad++

I found a workaround here.

You have to edit langs.xml. On Windows, it is located in %appdata%\notepad++. Change < Language name="normal" ext="txt"/> to < Language name="normal" ext=""/>. Then set your default language to Normal Text.

Alternately, you could change a language you don't use if you wish to keep Normal Text for .txt files. I chose makefile because I don't use it and there is only one setting for # comments.