specify available attributes for update

app/models/spree/digital.rb

@@ -6,6 +6,8 @@ module Spree
     has_attached_file :attachment, :path => ":rails_root/private/digitals/:id/:basename.:extension"
   
     # TODO: Limit the attachment to one single file. Paperclip supports many by default :/
+
+    attr_accessible :variant_id, :attachment
       
   end
 end