client.html

<input type="file">

<script>
var el = document.querySelector('input[type="file"]');
el.addEventListener('change', function(event) {
  let files = e.target.files;
  for (let i = 0; i < files.length; i++) {
    let file = files[i];
    var xhr = new XMLHttpRequest();
    // set the url to wherever you meteor app is running
    xhr.open('PUT', 'http://localhost:3000/file', true);
    xhr.onload = (event) => {
      console.log('done uploading!');
    };
    xhr.upload.onprogress = (event) => {
      let percent = 100 * (event.loaded / event.total);
      console.log(percent+'% uploaded');
    };
    xhr.send(file);
});
</script>

server.js

import { Meteor } from 'meteor/meteor';

WebApp.connectHandlers.use('/file', (req, res) => {
    res.setHeader("Access-Control-Allow-Methods", "PUT");
    res.setHeader("Access-Control-Allow-Origin", "*");
    res.setHeader("Access-Control-Allow-Headers", "Content-Type");

    if (req.method === 'OPTIONS') {
        res.writeHead(200);
        res.end();
        return;
    } else if (req.method === 'PUT') {
        if (!req.headers['content-type'].startsWith('image')) {
            res.writeHead(400);
            res.end();
        }
        
        let getFile = Meteor.wrapAsync(done => {
            let chunks = [];
            req.on('readable', () => {
                chunks.push(req.read());
            });
            req.on('end', () => {
                done(undefined, Buffer.concat(chunks));
            });
        });

        let buffer = getFile();

        res.writeHead(200);
        res.end();
    }
});