mail/internal/mail/message.go
func SetHeader(h map[string][]string) FileSetting {
	return func(f *file) {
		for k, v := range h {
			f.Header[k] = v
		}
	}
}
func SetHeader(h map[string][]string) FileSetting {
	return func(f *file) {
		for k, v := range h {
			f.Header[k] = v
		}
	}
}
// SetHeader is a file setting to set the MIME header of the message part that
// contains the file content.
//
// Mandatory headers are automatically added if they are not set when sending
// the email.