Remove hack to allow IO files in Active Storage

We were using this hack in order to allow `File.new` attachments in
tests files. However, we can use the `fixture_file_upload` helper
instead.

Just like it happened with `file_fixture`, this helper method doesn't
work in fixtures, so in this case we're using `Rack::Test::UploadedFile`
instead.

spec/shared/models/document_validations.rb

@@ -22,7 +22,7 @@ shared_examples "document validations" do |documentable_factory|
   it "is valid for all accepted content types" do
     acceptedcontenttypes.each do |content_type|
       extension = content_type.split("/").last
-      document.attachment = File.new(file_fixture("empty.#{extension}"))
+      document.attachment = fixture_file_upload("empty.#{extension}")
 
       expect(document).to be_valid
     end