Upload multipart file with ajax

file_upload_form.html.erb

<%= form_tag project_assets_path(project), multipart: true, class: 'upload-form' do %>
  <%= file_field_tag 'file' %>
  <%= submit_tag 'Upload' %>
<% end %>

file_upload_form.js

$('.upload-form').on('submit', function(event) {
  event.preventDefault();

  var form = $(event.target);
  var uploadUrl = form.attr('action');
  var formData = new FormData(event.target)

  $.post({
      url: uploadUrl,
      data: formData,
      processData: false,
      contentType: false
    })