// meanwhile in code
public class SessionFileStore
{
   public byte[] Content { get; set; }
   public string FileName { get; set; }
   public string ContentType { get; set; }
}

using System;
using System.Web;
using System.IO;
using System.Web.SessionState;
using System.Collections.Generic;
public class UploadCS : IHttpHandler, IRequiresSessionState {
    public void ProcessRequest (HttpContext context) {
        context.Response.ContentType = "text/plain";
        context.Response.Expires = -1;
        try
        {
            var postedFile = context.Request.Files["Filedata"];  // +
            if(postedFile != null && postedFile.ContentLength > 0) // +
            { // +
               var buffer = new byte[postedFile.ContentLength]; // +
               postedFile.InputStream.Read(buffer, 0, file.ContentLength); // +
               var files = context.Session["Files"] as List<SessionFileStore>; // *
               var fileStore = new SessionFileStore { 
                  Content = buffer, 
                  FileName = postedFile.FileName,
                  ContentType = postedFile.ContentType
               }; // +
               files.Add(fileStore); // *
               context.Response.Write(fileStore.FileName); // *
               context.Response.StatusCode = 200;
            } // +
        }
        catch (Exception ex)
        {
            context.Response.Write("Error: " + ex.Message);
        }
    }
 
    public bool IsReusable {
        get {
            return false;
        }
    }
}

protected void btnSend_Click(object sender, EventArgs e)
{
    using (var mailMessage = new MailMessage 
       {
          From = new MailAddress("user@gmail.com"),
          Subject = txtSubject.Text.Trim(),
          Body = txtBody.Text.Trim(),
          IsBodyHtml = true
       })
    {
        mailMessage.To.Add(new MailAddress(txtTo.Text.Trim()));
        var files = Session["Files"] as List<SessionFileStore>;
        foreach (var file in files)
        {
            mailMessage.Attachments.Add(new Attachment(new MemoryStream(file.Content), Path.GetFileName(file.FileName), file.ContentType));
        } 
        var smtp = new SmtpClient
           {
              Host = "smtp.gmail.com",
              EnableSsl = true,
              UseDefaultCredentials = true,
              Port = 587
           };
        var NetworkCred = new System.Net.NetworkCredential
           {
              UserName = mailMessage.From.Address,
              Password = "<Password>"
            };
        smtp.Credentials = NetworkCred;
        smtp.Send(mailMessage);
    }
    Response.Redirect(Request.Url.AbsoluteUri);
}

HttpContext.Current.Session["Files"] = new List<SessionFileStore>;

 [code=c#]
HttpContext.Current.Session["Files"] = new List<SessionFileStore>();