I am making a server with express to store some files,the site was deployed to heroku.And I found that the file saved on the server will disappear in a few hours.How can I fix that????

Using Packages: express express-fileupload

simple code:

main.js

app.post('/fileupload',(req,res)=>{
  if (!req.files) {
    return res.status(400).send("No files were uploaded.");
  }
  const file = req.files.myFile;
  const path = __dirname + "/files/" + file.name;

  file.mv(path, (err) => {
    if (err) {
      return res.status(500).send(err);
    }
    return res.send({ status: "success", path: path });
  });
})

terminal command(how i publish the server)

$ cd myproject
$ git add .
$ git commit -m "make it better"
$ git push heroku master

I know that I can use tools like S3,but the storage is limited,any free ways like installing npm packages or editing Heroku settings can solve this?