Express Multer validate request before uploading file

I am currently using multer-s3 (https://www.npmjs.com/package/multer-s3) to upload a single csv file to S3, i have it working this way:

var multer = require('multer');
var multerS3 = require('multer-s3');
var AWS = require('aws-sdk');

AWS.config.loadFromPath(...);
var s3 = new AWS.S3(...);

var upload = multer({
  storage: multerS3({
    s3: s3,
    bucket: 'my-bucket',
    metadata: function (req, file, cb) {
      cb(null, {fieldName: file.fieldname});
    },
    key: function (req, file, cb) {
      cb(null, Date.now().toString())
    }
  })
});

then it's routed like this:

app.route('/s3upload')
  .post(upload.single('data'), function(req, res) {

    // at this point the file is already uploaded to S3 
    // and I need to validate the token in the request.

    let s3Key = req.file.key;

  });

My question is, how can I validate the request object before Multer uploads my file to S3.

You can just chain one more middleware before the upload and then can check the token there

function checkToken(req, res) {
    // Logic to validate token
}

app.route('/s3upload')
  .post(checkToken, upload.single('data'), function(req, res) {

    // at this point the file is already uploaded to S3 
    // and I need to validate the token in the request.

    let s3Key = req.file.key;

  });