FileMaker natively does not support multiple email attachments. Your work around is to zip them and then attach them as a single attachment. You could even do this with one of the FileMaker plugins such as the free ScriptMaster from 360Works.
However, the more elegant way to do this is to get a FileMaker plugin that gives you more features like HTML formatting and attaching multiple emails. There are quite a few. I personally use CNS's SMTPit the most, but I think ScriptMaster will do it too and the license is free on it. There are probably a half dozen or more plugins that will do emails for you with multiple attachments. And they usually give you a lot more features too, which is nice.