Most mailers call these encoded files attachments or enclosures . Most e-mail programs contain attachment features that encode and decode files almost automatically. The person who receives a message containing attachments has to decode the files in order to use them. But just because you can use attachments doesn't mean you should.
Attachments create work.
The attachment must be decoded and then loaded into another application to be viewed. It might be worth it if the attachment is in a form needed for a report or project in progress, or if it is a graphic or program. If it's just words, maybe not. Attachments may not work. The recipient needs to use a mailer that accepts attachments and have software that can load the attached file.