How to tell paperclip not to delete the atachment if the edit form has the file field empty?

Question

So, I have this in my model:

# app/models/interior.rb
class Interior < ActiveRecord::Base
  belongs_to :casa

  has_attached_file :plano,
    :path => ":rails_root/public/planos_unidades/:styles/:id.:content_type_extension",
    :url => "/planos_unidades/:styles/:id.:content_type_extension",
    :styles => { :thumb => '98x98' },
    :convert_options => {
      :thumb => '-background "#f8f8f8" -gravity center -extent 98x98'
    }
end

# app/models/casa.rb
class Casa < ActiveRecord::Base
  has_many :interiors, :dependent => :destroy

  accepts_nested_attributes_for :interiors
end

And this in my create/edit form:

- form_for(:casa, :url => { :action => :save, :id => @casa.id }, :html => { :id => "casa_form", :multipart => true }) do |casa|
    - @casa.interiors.each_with_index do |interior, index|
        - fields_for "casa[interiors_attributes][#{index}]", interior do |interior|
            = interior.file_field(:plano)

Where index is a counter, in order to have multiple interiors in the same casa. I mean, when the user clicks "Add Interior" I render via AJAX the next:

- fields_for "casa[interiors_attributes][#{index}]", interior do |interior|
    = interior.file_field(:plano)

with an increasing value for index, of course.

Anyway, this works great for creating a casa and its respectives interiors, but when I want to edit it, I'm unable to retrieve the content for the :plano field, so when the user hit "Save", Paperclip decides it would be the hell of funny to delete the previously uploaded and saved :plano file! "Sorry" - it saids - ", but the :plano field was empty" @_@