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>