// adding a zip64 extra in the local header (too late anyway).
	var buf []byte
	if w.isZip64() {
		buf = make([]byte, dataDescriptor64Len)
	} else {
		buf = make([]byte, dataDescriptorLen)
	}
	b := writeBuf(buf)
	b.uint32(dataDescriptorSignature) // de-facto standard, required by OS X
	b.uint32(w.CRC32)
	if w.isZip64() {
		b.uint64(w.CompressedSize64)
		b.uint64(w.UncompressedSize64)
	} else {