Thanks Jason. I added the line and I'm not sure if it fixed the problem or not. I had to send out two messages, each with a file. Both times I did get auto replies ("out of the office") from people who were at the end of the alphabet of the recordset, so I know at least some messages were sent. But both times instead of going to the "your message was sent" page I got a Problem Loading Page error page which said "The connection to the server was reset while the page was loading."
Do you think that means the message was sent correctly, but there was just a problem with loading the confirmation page?
I looked at the page on PHP directives but, not being a programmer, I'm not sure where/how I would use that code. I'm fine with saying there's a limit to how big an attachment people can send with the current form - but I don't know what file size that would be as I don't recall seeing that in any FB instructions.