Much better to attach it as a portable document than write it in text
One, because that's literally mentioned in the email.
Two, the resume and cover letter gets circulated within organizations via email and/or print. Making it into an attachment makes it easy to print, and that too with the formatting/legibility retained.
Some organizations also have internal software/system which may require staff members to attach resume and cover letter. So, it becomes crucial that you share it in the form of a document than text.
You should consider attaching it in a portable format such as PDF (verses pages or document file), so that all the formatting, styling and hyperlinks are preserved. PDF is the most accessible format across any OS/platform, and is generally preferred by systems where a document is required to be uploaded.
Another benefit of PDF apart from retained formatting is that it couldn't be easily edited.
So then what can I put in the body of the email?
As far as email body goes, keep it concise and to the point. Don't leave it empty.
Hello Mr./Mrs. <Recruiter>,
Please find attached my Cover Letter and Resume for the position of <job-title>.
Kindly let me know if any additional information is required. Thank you for considering my profile for the position.