Fix detection for CFB files version 4. For #231

Previously the offset for the CLSID was searched for using v3 offsets.
This commit changes detection to check CFB version in order to
choose between v3(512) and v4(4096) offsets.
https://www.loc.gov/preservation/digital/formats/fdd/fdd000392.shtml

internal/magic/ms_office.go

@@ -190,15 +190,22 @@ func Msi(raw []byte, limit uint32) bool {
 //
 // http://fileformats.archiveteam.org/wiki/Microsoft_Compound_File
 func matchOleClsid(in []byte, clsid []byte) bool {
-	if len(in) <= 512 {
+	// Microsoft Compound files v3 have a sector length of 512, while v4 has 4096.
+	// Change sector offset depending on file version.
+	// https://www.loc.gov/preservation/digital/formats/fdd/fdd000392.shtml
+	sectorLength := 512
+	if len(in) < sectorLength {
 		return false
 	}
+	if in[26] == 0x04 && in[27] == 0x00 {
+		sectorLength = 4096
+	}
 
-	// SecID of first sector of the directory stream
+	// SecID of first sector of the directory stream.
 	firstSecID := int(binary.LittleEndian.Uint32(in[48:52]))
 
-	// Expected offset of CLSID for root storage object
-	clsidOffset := 512*(1+firstSecID) + 80
+	// Expected offset of CLSID for root storage object.
+	clsidOffset := sectorLength*(1+firstSecID) + 80
 
 	if len(in) <= clsidOffset+16 {
 		return false