I am trying to download files. The problem is that I do not know how to get its extension and they load me incorrectly, on the other hand if when I put its name I write the extension (knowing what it is) it is downloaded correctly. The problem is that in the app you don't know what type of file comes to you, you can get a pdf, a jpg, ... What I do is make a request from the NodeJS backend to AWS S3 to receive the file and I send it to the frontend of react.

Here is the backend code:

routerTrainer.get("/download-training", async (req, res) => {
  const { training_id } = req.headers;

  let training = await Training.findOne({
    where: { id: training_id },
  });

  if (training) {
    let readStream = getTrainingS3(`${training.file_id}`);
    readStream.on('close', () => {
      res.end()
    })
    readStream.pipe(res);
  }
});

And here is the clients Code:

const downloadTraining = async (id) => {
    fetch(`${serverPath}/download-training`, {
      headers: {
        training_id: id,
      },
    })
      .then((res) => res.blob())
      .then((blob) => {
        const link = document.createElement("a");
        link.href = URL.createObjectURL(blob);
        link.setAttribute("download", `here_is_where_I_need_to_put_extension_to_get_it_working`);
        document.body.appendChild(link);
        link.click();
      });
  };

Please if anyone has any ideas on how to do it, I would be very grateful for your help.