How to do ajax file upload in Rails 4?

Question

JavaScript

We have the following form:

<%= form_for @model,  :remote => true, :html => {:multipart => true} do |f| %>
    <%= f.text_field :title, :length => 255, :class=>'character-counter', :required=>true %>
...........
    <%= f.file_field :thumb %>
    <%= f.button :type=>'submit' %>
<%-end %>

The form works as expected, files are uploaded to the server (using the remotipart gem) via the iFrame method. But, if the response is not 200, but something like 422 (for example, validation errors), then the selected files in the input are reset, and you have to re-select files for uploading to the server, which, in fact, is not acceptable.
How to solve the problem? (possibly not using remotipart)