mail/internal/mail/message.go
func (m *Message) Embed(filename string, settings ...FileSetting) {
	m.embedded = m.appendFile(m.embedded, fileFromFilename(filename), settings)
}
func (m *Message) Embed(filename string, settings ...FileSetting) {
	m.embedded = m.appendFile(m.embedded, fileFromFilename(filename), settings)
}
// Embed embeds the images to the email.