Attach Files With An Email?

I have a form on which anyone can send me his/her information. Now i want to add 2 or 3 file upload options so that anyone from the website can send me their files along with the information. For getting information i am using this code (in button click) which works fine for me:

using System;
using System.Net.Mail;
using System.Text;
using System.Web.UI;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
protected void btnMyButton_Click(object sender, EventArgs e)
{
if (Page.IsValid)
{
try
{
string enquiry = txtEnquiry.Text.Trim();
if (String.IsNullOrEmpty(enquiry))
return;
string emailTitle = GetLocaleResourceString("MyTopic.EmailTitle");
string fullName = txtFullName.Text.Trim();
string email = txtEmail.Text.Trim();
string subject = string.Format("{0}. {1}", SettingManager.StoreName, emailTitle);
var body = new StringBuilder();
body.AppendFormat("<hr />{0} ({1})<hr />", emailTitle, DateTime.Now.ToShortDateString());
body.AppendFormat("<b>{0}:</b> {1}<br />", GetLocaleResourceString("MyTopic.FullName"), fullName);
body.AppendFormat("<b>{0}:</b> {1}<br />", GetLocaleResourceString("MyTopic.E-MailAddress"), email);
body.Append("<br />");
body.AppendFormat("<b>{0}:</b><br /> {1}<br />", GetLocaleResourceString("MyTopic.Enquiry"), MessageManager.FormatContactUsFormText(enquiry));
var from = new MailAddress(email, fullName);
var emailAccount = MessageManager.DefaultEmailAccount;
string body2 = body.ToString();
//required for some SMTP servers
if (SettingManager.GetSettingValueBoolean("Email.UseSystemEmailForContactUsForm"))
{
from = new MailAddress(emailAccount.Email, emailAccount.DisplayName);
body2 = string.Format("<b>From</b>: {0} - {1}<br /><br />{2}", Server.HtmlEncode(fullName), Server.HtmlEncode(email), body2);
}
var to = new MailAddress(emailAccount.Email, emailAccount.DisplayName);
MessageManager.InsertQueuedEmail(5, from, to, string.Empty, string.Empty, subject, body2,
DateTime.UtcNow, 0, null, emailAccount.EmailAccountId);
pnlResult.Visible = true;
pnlRequestQuote.Visible = false;
}

Now I added 3 file upload options in the page, now when i try to add code in .cs file i get errors. This is what i am trying to add (in BOLD)

protected void btnMyButton_Click(object sender, EventArgs e)
{
if (Page.IsValid)
{
try
{
string enquiry = txtEnquiry.Text.Trim();
if (String.IsNullOrEmpty(enquiry))
return;
string emailTitle = GetLocaleResourceString("MyTopic.EmailTitle");
string fullName = txtFullName.Text.Trim();
string email = txtEmail.Text.Trim();
string subject = string.Format("{0}. {1}", SettingManager.StoreName, emailTitle);
var body = new StringBuilder();
body.AppendFormat("<hr />{0} ({1})<hr />", emailTitle, DateTime.Now.ToShortDateString());
body.AppendFormat("<b>{0}:</b> {1}<br />", GetLocaleResourceString("MyTopic.FullName"), fullName);
body.AppendFormat("<b>{0}:</b> {1}<br />", GetLocaleResourceString("MyTopic.E-MailAddress"), email);
body.Append("<br />");
body.AppendFormat("<b>{0}:</b><br /> {1}<br />", GetLocaleResourceString("MyTopic.Enquiry"), MessageManager.FormatContactUsFormText(enquiry));
//'Attach the file
if (FileUpload2.HasFile)
{
body.Attachments.Add(new Attachment(AttachmentFile.PostedFile.InputStream, AttachmentFile.FileName));
body.Attachments.Add(new Attachment(FileUpload1.PostedFile.InputStream, FileUpload1.FileName));
body.Attachments.Add(new Attachment(FileUpload2.PostedFile.InputStream, FileUpload2.FileName));
}
else
{
body.Attachments.Add(new Attachment(AttachmentFile.PostedFile.InputStream, AttachmentFile.FileName));
body.Attachments.Add(new Attachment(FileUpload1.PostedFile.InputStream, FileUpload1.FileName));
}
var from = new MailAddress(email, fullName);
var emailAccount = MessageManager.DefaultEmailAccount;
string body2 = body.ToString();
//required for some SMTP servers
if (SettingManager.GetSettingValueBoolean("Email.UseSystemEmailForContactUsForm"))
{
from = new MailAddress(emailAccount.Email, emailAccount.DisplayName);
body2 = string.Format("<b>From</b>: {0} - {1}<br /><br />{2}", Server.HtmlEncode(fullName), Server.HtmlEncode(email), body2);
}
var to = new MailAddress(emailAccount.Email, emailAccount.DisplayName);
MessageManager.InsertQueuedEmail(5, from, to, string.Empty, string.Empty, subject, body2,
DateTime.UtcNow, 0, null, emailAccount.EmailAccountId);
pnlResult.Visible = true;
pnlRequestQuote.Visible = false;
}

I get this error message:

System.Text.StringBuilder' does not contain a definition for 'Attachments' and no extension method 'Attachments' accepting a first argument of type 'System.Text.StringBuilder' could be found (are you missing a using directive or an assembly reference?)