FM currently only allows one file attachment natively.
You can use a plugin to attach multiple attachments. You can also use an email service like SendGrid that has an API to allow multiple attachments (I prefer this method).
Other alternatives are to export all files and compress them into a zip with a plugin and send the single file. You could also create a PDF and send one PDF (not useful in all cases).
You can switch to MBS FileMaker Plug-in for sending emails.
Using SMTP via CURL functions. Check SendMail functions to build email with/without HTML, multiple attachments and properly encoded names & texts. Tutorial: Sending emails in FileMaker with MBS Plugin
We can even configure it for Outlook 365, Google mail and others. No problem there.
Or if you like to send via Outlook, please check our new SendMail functions in 7.2 version of our plugin. They can send via Outlook, Thunderbird or other MAPI clients, independent of 32/64bit issues and with a few more options. see Send emails via Outlook