builder.go
func (p MailBuilder) AddAttachment(b []byte, contentType string, fileName string) MailBuilder {
	part := NewPart(contentType)
	part.Content = b
	part.FileName = fileName
	part.Disposition = cdAttachment
	p.attachments = append(p.attachments, part)
	return p
}
func (p MailBuilder) AddAttachment(b []byte, contentType string, fileName string) MailBuilder {
	part := NewPart(contentType)
	part.Content = b
	part.FileName = fileName
	part.Disposition = cdAttachment
	p.attachments = append(p.attachments, part)
	return p
}
// AddAttachment returns a copy of MailBuilder that includes the specified attachment.