mail/internal/mail/message.go
func (m *Message) Attach(filename string, settings ...FileSetting) {
	m.attachments = m.appendFile(m.attachments, fileFromFilename(filename), settings)
}
func (m *Message) Attach(filename string, settings ...FileSetting) {
	m.attachments = m.appendFile(m.attachments, fileFromFilename(filename), settings)
}
// Attach attaches the files to the email.