Fix SaveAs dialog "Append extension" option not working issue

Fix the regression introduced in commit:
f884a39dd4

Fix https://community.notepad-plus-plus.org/topic/26235/notepad-v8-7-1-release-candidate/4
Close #15745

PowerEditor/src/WinControls/OpenSaveFileDialog/CustomFileDialog.cpp

@@ -505,7 +505,7 @@ private:
 		expandEnv(fileName);
 		bool nameChanged = transformPath(fileName);
 		// Update the controls.
-		if (doesDirectoryExist(getAbsPath(fileName).c_str()))
+		if (!doesDirectoryExist(getAbsPath(fileName).c_str()))
 		{
 			// Name is a file path.
 			// Add file extension if missing.