@GetMapping("/download")
@ResponseBody
public ResponseEntity serveFile(@RequestParam String filename) throws IOException{
    filename=new String(filename.getBytes(),"utf-8");//有没有这句话
    Resource file=new FileSystemResource("D:\\workspace\\idea_me\\demo\\upload-dir"+File.separator+ filename);
    HttpHeaders headers = new HttpHeaders();
    headers.add("Cache-Control", "no-cache, no-store, must-revalidate");
    headers.add("Content-Disposition", String.format("attachment; filename=\"%s\"", file.getFilename()));
    headers.add("Pragma", "no-cache");
    headers.add("Expires", "0");
    return ResponseEntity
            .ok()
            .headers(headers)
            .contentLength(file.contentLength())
            .contentType(MediaType.parseMediaType("application/octet-stream;charset=utf-8"))
            .body(new InputStreamResource(file.getInputStream()));
}