Header.txt text missing in repeatable case

Can you help me troubleshoot this SmartList problem? I searched the smartlist archives but couldn't find reference to this...

Problem: Text in the header.txt file is -not- included in an email if a file is attached to the email.

Additional information: * The problem does not happen if the email does not include any attachments.

* The proper SmartList files are set up (header.txt, rc.custom and rc.local.s20) correctly because the header text does display in emails without attachments.

I don't know where to start troubleshooting this. I just started using the basic features of SmartList for 'announcement-only' emails to a mailing list. Can you point me to where the cause might be?

Thanks in advance.

Abbo