code: func (m *MailYak) Attach(name string, r io.Reader) {
  	m.attachments = append(m.attachments, attachment{
  		filename: name,
  		content:  r,
  		inline:   false,
  	})
  }
docs: // Attach adds the contents of r to the email as an attachment with name as the
  // filename.
  //
  // r is not read until Send is called and the MIME type will be detected
  // using https://golang.org/pkg/net/http/#DetectContentType