length := typ.RawByteLen()
		if length == 0 || len(b) < length {
			break
		}
		cs := base64.StdEncoding.EncodeToString(b[:length])
		b = b[length:]
		if typ.Is(ChecksumMultipart) {
			t, n := binary.Uvarint(b)
			if n < 0 {
				break
			}
			cs = fmt.Sprintf("%s-%d", cs, t)
			b = b[n:]
			if part > 0 {
				cs = ""
			}
			if typ.Is(ChecksumIncludesMultipart) {
				wantLen := int(t) * length