Fake async file upload using an iframe.

client.js

$( function(){

  var form = $('<form>');
  var iframe = $('<iframe name="upload_iframe">');
  var file_upload = $('.file-upload');

  // Form

  form.attr("target", "upload_iframe");
  form.attr("action", '/upload');
  form.attr("method", "post");
  form.attr("enctype", "multipart/form-data");
  form.attr("encoding", "multipart/form-data");

  // File upload

  file_upload.change( function(){

    iframe.unbind('load').bind('load', function() {
      var response = iframe.contents().find('body').text();
    });

    form.submit();

  });

  // Iframe

  iframe.attr('id', 'upload_iframe');
  iframe.hide();

  // Add elements

  file_upload.after( iframe );
  file_upload.after( form ); 
  form.append( file_upload );

} );

express-example.js

var contentType = req.headers.accept.indexOf('application/json') !== -1 ? 'application/json' : 'text/plain';
res.header('Content-Type', contentType);
res.send(JSON.stringify(data));

index.html

<input type='file' class='file-upload' name='image' />