envelope.go
func matchHTMLBodyPart(p *Part) bool {
	return p.ContentType == ctTextHTML && p.Disposition != cdAttachment
}
func matchHTMLBodyPart(p *Part) bool {
	return p.ContentType == ctTextHTML && p.Disposition != cdAttachment
}
// Used by Part matchers to locate the HTML body.  Not inlined because it's used in multiple places.