Paperclip nested resources problem

My models:

class Country < ActiveRecord::Base
  has_many :regions
  has_many :assets, :dependent => :destroy
  accepts_nested_attributes_for :assets
end

class Region < ActiveRecord::Base
  belongs_to :country
  has_many :appartments

  has_many :assets, :dependent => :destroy
  accepts_nested_attributes_for :assets
end

class Asset < ActiveRecord::Base
  belongs_to :region
  belongs_to :country
  has_attached_file :image,
    :styles => {
      :thumb=> "100x100>",
      :small => "300x300>",
      :large => "600x600>"
  }
end

The region _form

<% form_for([@country, @region]) do |f| %>
    <%= f.label :title %><br />
    <%= f.text_field :title %>
    <%= f.label :content %><br />
    <%= f.text_area :content %>
<%= f.fields_for :assets do |asset| %>
           <% if asset.object.new_record? %>
                 <%= asset.file_field :image %>

controller:
@country = Country.find(params[:country_id])
@region = @country.regions.find(params[:id])
  5.times { @region.assets.build }

When uploading an image only the region_id is filled in and not the
paperclip attributes.

What i am doing wrong?

Grtz..remco