upload-file-server.js

var express = require('express')
var multer = require('multer')

var storage = multer.memoryStorage()
var upload = multer({ storage: storage })

var app = express()

app.post('/upload', upload.single('users.csv'), function (req, res, next) {
  if (!req.file) {
    return res.status(400).send('No file uploaded')
  }

  // you can choose to restrict file types
  if (req.file.mimetype !== 'text/csv') {
    return res.status(400).send('File type must be csv')
  }
  
  // file is ready, access it in req.file
  // ...
  // ...

})

app.listen(3000)