Description of the Issue

Steps to Reproduce the Issue

1. Preferences > Default Directory
2. Select 3rd radio button, specify a path to an empty directory without any files
3. File > New
4. File > Save As
5. Ensure the "Append Extension" is checked ☑
6. Change the name to "test" (no extension)
7. Press "Save"

Expected Behavior

File is saved as "test.txt".

Actual Behavior

File is saved as "test" without an extension.

Debug Information

Notepad++ v8.1.2 (64-bit)
Build time : Jul 16 2021 - 01:59:50
Path : C:\Program Files\Notepad++\notepad++.exe
Command Line :
Admin mode : OFF
Local Conf mode : OFF
Cloud Config : OFF
OS Name : Windows 10 Enterprise (64-bit)
OS Version : 2009
OS Build : 19043.1165
Current ANSI codepage : 1252
Plugins : DSpellCheck.dll HexEditor.dll JSMinNPP.dll MarkdownViewerPlusPlus.dll mimeTools.dll NppConverter.dll NppEditorConfig.dll NppExport.dll NppMarkdownPanel.dll NppMenuSearch.dll NppTags.dll PythonScript.dll XMLTools.dll