How to attatch file with a mail ? Is there any problem in config file?

try
           {
               //FileStream file = null;

               // //Read the file. This appears to be the offending line
               //file = File.OpenRead(path);
               //byte[] Content = new byte[file.Length];
               //file.Read(Content, 0, (int)file.Length);
               //file.Close();
               //file.Dispose();
               //Label1.Text = Content.ToString();

               MailMessage mail = new MailMessage();
               //m.ail.To.Add("anzaman_edi@yahoo.com");
               mail.To.Add("imran3may@gmail.com");
               mail.To.Add("shi01715@yahoo.com");
               //mail.To.Add("zahir@eminence-bd.org");
               mail.From = new MailAddress("shishir64092@gmail.com");
               mail.Subject = "Send Email by asp.net code using google or gmail smtp server";

               string Body = "Hi, I am testing Email function in asp.net" +
                             " using google or gmail smtp server"+
                             " and next time I will send you a document from my .aspx file";
               mail.Body = Body;
               Attachment at = new Attachment("D:\\Text");
               mail.Attachments.Add(at);

               mail.IsBodyHtml = true;

               SmtpClient smtp = new SmtpClient("localhost",25);
               smtp.Host = "smtp.gmail.com"; //Or Your SMTP Server Address
               smtp.Credentials = new System.Net.NetworkCredential
                    ("shishir64092@gmail.com", "abcdef");
               //Or your Smtp Email ID and Password
               smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
               smtp.EnableSsl = true; ;

               smtp.Send(mail);
               Label1.Text = "Mail Send...";
           }
           catch (Exception ex)
           {
               Label1.Text = ex.Message;
           }

<?xml version="1.0"?><configuration>
	<system.net>
		<mailSettings>
			<smtp deliveryMethod="PickupDirectoryFromIis">
				<network defaultCredentials="true" host="localhost" port="25"/>
			</smtp>
		</mailSettings>
	</system.net>
	<system.web>
		<compilation debug="true">
			<assemblies>
				<add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
				<add assembly="System.Web.Abstractions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
				<add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
				<add assembly="System.Data.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
				<add assembly="System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
				<add assembly="System.Data.Services.Client, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
				<add assembly="System.Data.Services.Design, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
				<add assembly="System.Data.Entity, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
				<add assembly="System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/></assemblies></compilation></system.web></configuration>