builder.go
func (p MailBuilder) AddInline(
	b []byte,
	contentType string,
	fileName string,
	contentID string,
) MailBuilder {
	part := NewPart(contentType)
	part.Content = b
	part.FileName = fileName
	part.Disposition = cdInline
	part.ContentID = contentID
	p.inlines = append(p.inlines, part)
	return p
}
func (p MailBuilder) AddInline(
	b []byte,
	contentType string,
	fileName string,
	contentID string,
) MailBuilder {
	part := NewPart(contentType)
	part.Content = b
	part.FileName = fileName
	part.Disposition = cdInline
	part.ContentID = contentID
	p.inlines = append(p.inlines, part)
	return p
}
// AddInline returns a copy of MailBuilder that includes the specified inline.  fileName and
// contentID may be left empty.