Calling code behind function from a html button

HTML button click call code behind in asp.net

<input type="button" id="btnuplod" runat="server" value="Upload" onclick="__doPostBack('btnSubmit','Submit');">

Code Behind

 

protected void Page_Load(object sender, EventArgs e)

{

   if (IsPostBack)

   {

      if (Convert.ToString(Request.Form["__EVENTARGUMENT"]) == "Submit")

      {

          btnuplod_Click();//submit action here

      }

   }

}

Your Function

Public void btnuplod_Click()

{

      if (FileUploadPDF.HasFile)

        {

            string fileName = Server.HtmlEncode(FileUpload1.FileName);

            string extension = System.IO.Path.GetExtension(fileName);

            if (extension == ".pdf")

            {

            }

            else

            {

                lblerror.InnerText = "Please upload PDF Files Only";

            }

        }

}