images_controller.rb

  def create
    @post = Post.find_by_slug(params[:post_id])
    @image = @post.images.build(params[:image])
    if @image.save
      respond_to do |format|
        format.js
      end
    end
  end

upload_multiple.js

jQuery.ajaxSetup({
  'beforeSend': function(xhr) {
    xhr.setRequestHeader("Accept", "text/javascript")
  }
})

$('#image_image').change(function() {

  jQuery.each($('#image_image')[0].files, function(i, file) {
    var data = new FormData();
    data.append("image[image]", file);

    $.ajax({
      url: 'images',
      data: data,
      cache: false,
      contentType: false,
      processData: false,
      type: 'POST',
    });

  });

});