// Read compression metadata preserved in the init multipart for the decision.
	_, isCompressed := mi.UserDefined[ReservedMetadataPrefix+"compression"]
	// Compress only if the compression is enabled during initial multipart.
	var idxCb func() []byte
	if isCompressed {
		wantEncryption := crypto.Requested(r.Header) || isEncrypted
		s2c, cb := newS2CompressReader(reader, actualPartSize, wantEncryption)
		idxCb = cb