import org.springframework.web.util.UriUtils;
// HttpServletResponse response;
final String CONTENT_DISPOSITION_FORMAT = "attachment; filename=\"%s\"; filename*=UTF-8''%s";
String fileName = "日本語ファイル名.txt";
String headerValue =
String.format(CONTENT_DISPOSITION_FORMAT,
fileName,
UriUtils.encode(fileName, StandardCharsets.UTF_8.name()));
response.setHeader("Content-Disposition", "attachment; filename=" + headerValue);