Старый, но ...
<%= form_for(DemoUpload.new, :remote => true ,:url =>"/demo_upload") do |f| %>
изменить на:
<%= form_for(DemoUpload.new,:html => {:id=>"Demo-upload", :'data-type' => :html,:remote => true,:multipart => true } ,:url =>"/demo_upload") do |f| %>
Сценарий:
$("#Demo-upload").bind("ajax:complete", function(e, data, status, error){
if (data.status === 200 || data.status === 201) {
//success
}else {
// error
}
})