Simply list the title of the document and then include the description in parentheses. For example, you might write "Enclosures: Birth certificate (original plus two copies)." If you've enclosed original documents that you're going to need returned to you, make sure you mention that in the body of the letter.

An enclosure in a cover letter is a list of any additional documents you’ve included in your application. Typical enclosure documents include letters of recommendation, certificates, and written tests associated with the job application.