// Guess the magic values.
	magic := string(b.toUSTAR().magic())
	version := string(b.toUSTAR().version())
	trailer := string(b.toSTAR().trailer())
	switch {
	case magic == magicUSTAR && trailer == trailerSTAR:
		return formatSTAR
	case magic == magicUSTAR:
		return FormatUSTAR | FormatPAX
	case magic == magicGNU && version == versionGNU:
		return FormatGNU
	default:
		return formatV7
	}
}