You misunderstood what the employer meant by "plain text".

Plain text allows you to write your resume directly in the body of the letter, without using any attachments or TXT. Don't you need to add attachments if you use TXT?

Employers will receive a lot of resumes, and it is very troublesome to download and open the attachments of these resumes and read them again, so use this to invite people to submit resumes for their own convenience.

In short, just write it directly in the body of the letter.