part.go
func (p *Part) TextContent() bool {
	if p.ContentType == "" {
		// RFC 2045: no CT is equivalent to "text/plain; charset=us-ascii"
		return true
	}
	return strings.HasPrefix(p.ContentType, "text/") ||
		strings.HasPrefix(p.ContentType, ctMultipartPrefix)
}
func (p *Part) TextContent() bool {
	if p.ContentType == "" {
		// RFC 2045: no CT is equivalent to "text/plain; charset=us-ascii"
		return true
	}
	return strings.HasPrefix(p.ContentType, "text/") ||
		strings.HasPrefix(p.ContentType, ctMultipartPrefix)
}
// TextContent indicates whether the content is text based on its content type.  This value
// determines what content transfer encoding scheme to use.