The list of file types in the SaveAs dialog aren’t necessarily configurable. And having a software developer go and add every possible extension that anyone of the untold thousands of users might use, or might invent for their own use-case, is not something that can be practically done.
Besides, all you have to do to get the file extension you want is to type it yourself. Since you’re already typing the filename, it shouldn’t be that hard to type .txt+ as well… and probably easier than typing the filename without an extension, then dragging down the pulldown and hoping that Windows will apply the right extension based on that pulldown entry.
It appears you are using the “old style” dialog by disabling the ☐ Use new style dialog (without file extension feature & Unix style path capacity) in Settings > Preferences > Default Directory. I say this because you seem to have an expectation that the pulldown will set your extension for you. In my experience, this only happens in the old-style dialog. If you are using the old-style dialog, be wary of that “feature” from Windows… because whatever happens to come first in the list of extensions for a given dropdown is what Windows will apply; hence, if you type blah and apply c++ source file, there’s a good chance that you’ll save blah.h rather than blah.cpp, which is probably not what you want. I always use the new-style, because the old-style was obsoleted by Windows’s new-style dialog half a decade ago or more, and (more importantly) because the new-style encourages you to take control of your destiny by controlling extension yourself rather than relying on Windows to fill in the extension for you, and (most importantly to me) because it enables the tree-view to the left rather than just having the four or five icons for windows-preferred locations, and making it more clicks to find the location you want.
Also, assuming you installed Notepad++ normally, I thought there was usually a right-click menu on most files which allows you to choose “Open With Notepad++” or similar phrasing, even if it’s not directly associated with that file type. If not, you could use save the following as a .reg file,
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Classes\*\shell\Notepad++]
@="Edit With Notepad++"
"Icon"="\"C:\\Program Files\\notepad++\\notepad++.exe\""
[HKEY_CURRENT_USER\Software\Classes\*\shell\Notepad++\command]
@="\"C:\\Program Files\\notepad++\\notepad++.exe\" \"%1\""
(where you might need to use c:\\program files (x86)… instead, if you’re 32-bit)
That way, you can use the default associated program whenever you want by the normal double-click, but use Notepad++ by using the right-click menu for any file type
