Unfortunately, you cannot add an attachment to a message without embedding a link in the body of the message. You can only insert attachment links to files hosted on the SimplyCast site, or create links to files hosted on your own web server.
Attaching a file to a message makes its file size significantly larger, which will make sending times increase substantially. You would also risk filling up a recipient's mailbox or having your email rejected because its file size is too large. There are even situations where some mail servers will remove attachments from messages that are received. Creating a link to a file allows the recipient to quickly view or download it.