SaveFileDialog从设置的文件名中删除扩展名

我有下面的应用程序。为了便于读者进行测试，我稍微修改了一下。我注意到，当我用扩展名设置文件名时，例如test.txt，文本扩展名被对话框删除。但是，我希望用户能够指定扩展，更重要的是，我希望能够设置扩展。破解它的一种方法是根据我的扩展修改过滤器。这是唯一的办法吗?

我使用的是vs2010 Express.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Globalization;
using System.IO;
using System.Windows;
namespace SpeedDating
{
    partial class Program
    {
         [STAThread]
        static void Main(string[] args)
        {
            Form form = new Form();
            form.WindowState = FormWindowState.Minimized;
            form.ShowInTaskbar = false;
            form.TopMost = true;
            form.Show();
            string filename = "test.txt";
            string ext = filename.Substring(filename.LastIndexOf('.'));
            SaveFileDialog dialog = new SaveFileDialog();
            dialog.Title = "SpeedDating App";
            dialog.RestoreDirectory = true;
            dialog.CheckFileExists = false;
            dialog.CheckPathExists = false;
            dialog.SupportMultiDottedExtensions = true;
            dialog.AddExtension = false;
            dialog.Filter = "All files (*.*)|*.*";
            dialog.FileName = DateTime.Now.ToString("yyyyMMdd") + ext;
            DialogResult result = dialog.ShowDialog();
            if (result == DialogResult.OK && dialog.FileName != "")
            {
                try
                {
                    FileStream outfs = File.Create(dialog.FileName);
                    FileStream infs = File.Open(filename, FileMode.Open);
                    infs.CopyTo(outfs);
                    infs.Close();
                    outfs.Close();
                    MessageBox.Show(form, "Copied file.");
                }
                catch (NotSupportedException ex)
                {
                    MessageBox.Show(form, "Probably removed the original file.");
                }
            }
            else if (result != DialogResult.Cancel)
            {
                MessageBox.Show(form, "No path found to write to.");
            }
            form.Close();
        }
    }
}

        string filename = "test.xyz";
        SaveFileDialog dialog = new SaveFileDialog();
        dialog.Title = "SpeedDating App";
        dialog.RestoreDirectory = true;
        dialog.CheckFileExists = false;
        dialog.CheckPathExists = false;
        dialog.SupportMultiDottedExtensions = true;
        dialog.Filter = "All files (*.*)|*.*";
        dialog.DefaultExt = System.IO.Path.GetExtension(filename);
        if (dialog.DefaultExt.Length > 0)
        {
            dialog.AddExtension = true;
            dialog.Filter = dialog.DefaultExt + " files (*." + dialog.DefaultExt + ")|*." + dialog.DefaultExt + "|" + dialog.Filter;
            dialog.FilterIndex = 0;
        }
        dialog.FileName = DateTime.Now.ToString("yyyyMMdd");
        DialogResult result = dialog.ShowDialog();
        if (result == DialogResult.OK && dialog.FileName != "")
        {
            Console.WriteLine(dialog.FileName);
        }