sizes = append(sizes, []int{4 << 20, 64 << 20}...)
	}
	maxSize := 2 * (sizes[len(sizes)-1] + 8)
	if len(bmbuf) < maxSize {
		bmbuf = make([]byte, maxSize)
	}

	for _, n := range sizes {
		for _, off := range []int{0, 1, 4, 7} {
			buf1 := bmbuf[off : off+n]
			buf2Start := (len(bmbuf) / 2) + off
			buf2 := bmbuf[buf2Start : buf2Start+n]
			buf1[n-1] = 'x'
			buf2[n-1] = 'x'

		// if modification time is the only timestamp being encoded.
		var mbuf [9]byte // 2*SizeOf(uint16) + SizeOf(uint8) + SizeOf(uint32)
		mt := uint32(fh.Modified.Unix())
		eb := writeBuf(mbuf[:])
		eb.uint16(extTimeExtraID)
		eb.uint16(5)  // Size: SizeOf(uint8) + SizeOf(uint32)
		eb.uint8(1)   // Flags: ModTime
		eb.uint32(mt) // ModTime
		fh.Extra = append(fh.Extra, mbuf[:]...)
	}

	var (
		ow io.Writer
		fw *fileWriter