sp_send_dbmail with attachment SQL Server 2008, connecting via SQL Server Authentication.

I have a stored procedure in DatabaseA which calls sp_send_dbmail in msdb to send an email with a file attachment. The file is on the db server, not on a remote fileshare.

The SQL Server account being used is not sysadmin, but does belong to the DatabaseMailUserRole in msdb.

Sending an email without an attachment is fine, but when an attachment is present I get an error:


  The client connection security context could not be impersonated. Attaching files require an integrated client login


There are a few articles/posts about this out there, but some seem to say conflicting things. I've been looking into impersonation, and one thing that does work is in the stored procedure in DatabaseA, to do the following:

EXECUTE AS LOGIN = 'sa' -- or any account with sysadmin privileges
EXECUTE msdb..sp_send_dbmail ....
REVERT


I wasn't expecting this to work as I thought to send attachments, you needed to use Windows Authentication. However it does work, but it means the lower privileged SQL Server account needs to be granted permission to IMPERSONATE the sa (or other sysadmin account).

Doing my due diligence as a dev before unleashing a DBA's nightmare into the wild...

My question is: what is a good/safe way of allowing a user authenticated via SQL Server (non sysadmin) to send email attachments from the local db server disk without opening up a security hole?

Update:

Re: Credentials

I've created a new Windows Login, created credentials for that account via SSMS, mapped those credentials to my limited privileges SQL Server account. I get the error:


  Msg 22051, Level 16, State 1, Line 0
  The client connection security context could not be impersonated.
  Attaching files require an integrated client login


I must be missing something!
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.