sp_send_dbmail with attachment
You need to use Credentials:


  A credential is a record that contains the authentication information
  (credentials) required to connect to a resource outside SQL Server.
  This information is used internally by SQL Server. Most credentials
  contain a Windows user name and password.
  
  The information stored in a credential enables a user who has
  connected to SQL Server by way of SQL Server Authentication to access
  resources outside the server instance. When the external resource is
  Windows, the user is authenticated as the Windows user specified in
  the credential. 


This way you can associate your SQL login that sends the mail with an NT credential that has file access to the attachment.