我在heroku上部署了我的nodejs和react应用程序，但是multer upload在localhost上工作，但在生产中不工作，它给出错误Access to fetch has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource，这是不正确的

const cors = require("cors");

app.use(cors());
app.use("/files", express.static(path.join(__dirname, "public/files")));

const storage = multer.diskStorage({
  destination: function (req, file, cb) {
    cb(null, "public/files");
  },
  filename: function (req, file, cb) {
    cb(null, Date.now() + "-" + file.originalname);
  },
});
const upload = multer({ storage: storage }).array("file");

app.post("/upload", function (req, res) {
  upload(req, res, function (err) {
    if (err instanceof multer.MulterError) {
      return res.status(500).json(err);
    } else if (err) {
      return res.status(500).json(err);
    }
    return res.status(200).send("files have been uploaded successfully");
  });
});

app.listen(process.env.PORT || 8000, () => {
  console.log("listening at port 8000");
});

通过react请求发送到/upload，其中包含所需文件：

const [files, setFiles] = useState([]);

  const handleCareer= ()=>{
      const data = new FormData();
      const pdf = `${Date.now()}${files[0]?.name}`;
      const image = `${Date.now()}${files[1]?.name}`;

      for (let x = 0; x < files.length; x++) {
        data.append("file", files[x]);
      }

      career.resume = pdf;
      career.resumeImage = image;
      await fetch("https://shoushapi.herokuapp.com/upload", {
        method: "POST",
        body: data,
      });
}

return (
   <form>
    ...

          <div className="row">
            <label htmlFor="resumeFile" className="downloadImg">
              <AddAPhotoIcon className="postIcon" />
              <span>Upload resume</span>
            </label>

            <input
              type="file"
              accept="application/pdf,application/msword"
              id="resumeFile"
              name="file"
              className="upload"
              onChange={(e) => setFiles([...files, e.target.files[0]])}
            ></input>
          </div>

          <div className="row">
            <label htmlFor="resumeImage" className="downloadImg">
              <AddAPhotoIcon className="postIcon" />
              <span>Upload resume image</span>
            </label>

            <input
              type="file"
              accept=".png,.jpeg,.jpg"
              id="resumeImage"
              className="upload"
              onChange={(e) => setFiles([...files, e.target.files[0]])}
              name="image"
            ></input>
          </div>

          <div className="row">
            <button type="button" className="btn" onClick={handleCareer}>
              Request
            </button>
          </div>
   </form>

代码在开发模式下运行良好，如果我将Access-Control-Allow-Origin更改为http://localhost:8000，并将fetch url更改为http://localhost:8000/upload，则通过在本地运行可以成功上传文件，这里有什么问题？