how to check validation in file uploading like .doc,.docx,.txt,.xls,.xlsx .with example in asp.net with c# language ,with code

Hey, this is just an example....

using System;
using System.IO;
using System.Collections;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;


public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        ArrayList list = new ArrayList();
        list.Add(".doc");
        list.Add(".docx");
        list.Add(".txt");
        list.Add(".xls");
        list.Add(".xlsx");


        string Extension = Path.GetExtension(FileUpload1.PostedFile.FileName);
        foreach (string s in list)
        {
            if (Extension == s)
            {
                Response.Write("File Type Not allowed. please choose some other file type");
                // Or you can write your logic here
            }
            else
            {
                // Your Code Logic Comes here
            }
        }
    }
}

You can write this code in the Button Click Event... Hope this helps you out..

Thank you!!!
Please mark this answer as accepted, if it helped!!!