Return document max file size in megabytes

The same way it's done for images.

We were converting the number of megabytes to bytes and then converting
it to megabytes again. Instead, we can leave it as it is and only
convert it to bytes when necessary (only one place).

spec/shared/models/document_validations.rb

@@ -2,7 +2,7 @@ shared_examples "document validations" do |documentable_factory|
   include DocumentablesHelper
 
   let!(:document)               { build(:document, documentable_factory.to_sym) }
-  let!(:maxfilesize)            { max_file_size(document.documentable.class) }
+  let!(:maxfilesize)            { document.documentable.class.max_file_size }
   let!(:acceptedcontenttypes)   { accepted_content_types(document.documentable.class) }
 
   it "is valid" do