Maintain a string element in your schema say for example "AttachmentString" and pass the same in Email activity "Attachment" tab
This will get the data from the element "AttachmentString" and pass on the same as attachment to the mail.
It is also important to pass on the right mime type. By default it will be "text/html;"
Follow below steps,
1: Add Email Activity
2: Give Details Under "General"
3: Goto Attachments
4: There you will see three tabs
Name is the name which you want for the attachment file. Use extensions also like .txt etc
MimeType you can give text/html; for plain texts
Value is used for the content of the attachment. Use expresion builder to fetch your variable and give under this tab.
Deploy and run. Attachments will start going through email.