That's quite a difficult question. Attachments are attached to an email from a users local machine, and whatever you want to attach is sitting on your Mendix server. Transferring a file from your server to a client machine, then attaching it to an email is tricky.
You can find many examples online if you search on non-Mendix related content. Try here and here. You can implement these as link button urls or JavaScript actions from Mendix.