Fixtures with Paperclip

I'm using Paperclip to store documents, but I can't find how to create fixtures of them, I wrote this :

<% doc = Document.create(:asset => File.open(Rails.root.join('spec', 'assets', 'image.png'))) %>
<%= part_event_image %>:
  asset_file_name: <%= doc.asset_file_name %>
  asset_content_type: <%= doc.asset_content_type %>
  asset_file_size: <%= doc.asset_file_size %>
  asset_updated_at: <%= doc.asset_updated_at %>
  documentable: party (Event) %>
<% end %>

But one I ran it, the document exist in the database, but it's not stored.

Should I store the document myself (write the file) ? Or is there an other way ?