How to add extension to a file without extension in C#

hello sir ,

i have a code to replace the extension but i want to add extension(.log) to those files without any extension

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim myFiles As String()

        myFiles = IO.Directory.GetFiles("D:\EXTENSIONFILE\", "*.txt")

        Dim newFilePath As String

        For Each filepath As String In myFiles

            newFilePath = filepath.Replace(".txt", ".log")

            System.IO.File.Move(filepath, newFilePath)

        Next

    End Sub