app/views/user/attachments/create.js.erb
<% if @attachment.new_record? %>
alert("Error: <%= j @attachment.errors.full_messages.uniq.join(', ').html_safe %>");
<% else %>
$("#attachments").append('<%= j render(@attachment) %>');
$("#<%= dom_id(@attachment) %>").hide();
$("#<%= dom_id(@attachment) %>").fadeIn();
<% end %>