I dont think you can send more than 1 email attachment natively out of FileMaker. You need to combine all the attchments together (which I believe you have to do seperately as a zip or a meged pdf etc). I'm not sure whether this functionality has changed in FileMaker 13 though.
The FM (all versions) Script command Send email does not support multiple attachments. There are ways to accomplish this.
1. Create the attachments and use an OS command to either combine them as one (for example concatenate PDF documents) or to compress them together as one file (for example StuffIt).
2. Use a plugin.
3. Use the URL send email command
4. Use the OS built-in Send email program, using either AppleScript or a batch file command file
5. Send using client and allow the User to attached additional attachments