Send email with Attachment in C#

public static string SalesNotifiesToProduction(string UserName, string company, string publication,  int noticeid,
                  string projectanme, string summary,string SpecialInstructions,string logo,string PEmail,string Email,string Phonenumber,
           string ResponseType,string EmailResponseTo,string Fax,string DirectURL,string PrintDates)
        {
            try
            {
                MailMessage mail = new MailMessage();
                mail.To.Add(PEmail);
                mail.Bcc.Add("my email");
               
                mail.From = new MailAddress(ConfigurationManager.AppSettings["MailFrom"].ToString());
                mail.Subject = "OPBN Notice #: " + noticeid + " | Type Set Request";
               
//// attachment code here //

/// code ends //

                string str = "";
                if (ResponseType == "Yes")
                {
                    string strFax = "";
                    if (Fax != null)
                    {
                       strFax=" , Fax: " + Fax;                        
                    }
                    string strEmail = "";
                    if (EmailResponseTo != null)
                    {
                        strEmail = " , Email: " + EmailResponseTo;
                    }
                    string strURL = "";
                    if (DirectURL != null)
                    {
                        strURL = " Contact URL: <a href="" + DirectURL + "">Click Here </a> ";
                    }
                    str =strURL + strEmail + strFax;
                }

                string Body = "Some text"; 
                mail.Body = Body;

                mail.IsBodyHtml = true;
                SmtpClient smtp = new SmtpClient(ConfigurationManager.AppSettings["mailserver"].ToString(), Convert.ToInt32(ConfigurationManager.AppSettings["port"]));
                if (Convert.ToBoolean(ConfigurationManager.AppSettings["developement"]))
                {
                    smtp.Credentials = new NetworkCredential(ConfigurationManager.AppSettings["username"].ToString(), ConfigurationManager.AppSettings["password"].ToString());
                }
                smtp.EnableSsl = Convert.ToBoolean(ConfigurationManager.AppSettings["EnableSsl"]);
                smtp.Send(mail);
                return "Sent Notification Success!";
            }
            catch (Exception ex)
            {
               return ex.Message.ToString();
            }
        }


this works perfectly fine and send email. But when i tried to attach a file using this code

 String filename = VirtualPathUtility.GetFileName(logo);


                if (filename != null && filename.Length > 0 )
                {
                    //Attachment file = new Attachment(filename);
                    //mail.Attachments.Add(file);

                    var attachment = new Attachment(filename);
                    mail.Attachments.Add(attachment);
                }