I originally thought the same thing. But no, it's not included inside the email file.
I checked two ways ...
i) By opening an email, that had an attachment, with notepad. There was no text that might have been an 'attachment'. Everything was simple human readable stuff. No block of hex code or whatever that might have been some kind of attachment.
ii) By checking the size of an email file, that had an attachment. The email was 3kb. The attachment was far larger. Hence, the email could not have contained the attachment.
Conclusion, no. The email file does not contain its attachment. The attachment must be located somewhere else on the hard drive.