Send Mail is limited to only a single attached file.This is a known limitation to how send mail works.
How did you attempt to attach more than one file?
I'm on windows, and can't figure out any method that would lead me to think that I had more than one attached file.
Thank you for your post, and I apologize for the late reply.
The answer provided by "PhilModJunk" is correct (Thank you). FileMaker Pro will only allow one file attachment in a Send Mail script. If you need to send more than one file attachment, then you need to compress/zip the files and attach the zip file.