Erro quando anexo o arquivo no e-mail

Question

 string App_Path = "";


            App_Data = "~/App_Data/10/CNA.html";

MemoryStream MS = new MemoryStream(App_Path.FileBytes);
Attachment anexo = new Attachment(MS, App_Path);
mailMessage.Attachments.Add(anexo);
objStreamReader.Close();

Error 32 'string' does not contain a definition for 'FileBytes' and no extension method 'FileBytes' accepting a first argument of type 'string' could be found (are you missing a using directive or an assembly reference?) C:\ARTSIM\SGSIM\ControledeUsuarios\MailingB2B.aspx.cs 341 61 ControledeUsuarios