part.go
func NewPart(contentType string) *Part {
	return &Part{
		Header:      make(textproto.MIMEHeader),
		ContentType: contentType,
	}
}
func NewPart(contentType string) *Part {
	return &Part{
		Header:      make(textproto.MIMEHeader),
		ContentType: contentType,
	}
}
// NewPart creates a new Part object.