Use documentables helper method within specs and document model.

spec/shared/models/document_validations.rb

@@ -3,6 +3,7 @@ shared_examples "document validations" do |documentable_factory|
   include DocumentablesHelper
 
   let!(:document)               { build(:document, documentable_factory.to_sym) }
+  let!(:documentable)           { document.documentable }
   let!(:maxfilesize)            { max_file_size(document.documentable) }
   let!(:acceptedcontenttypes)   { accepted_content_types(document.documentable) }