Only cache a single instance of uploaded attachments.

Prior to this fix, uploading an attachment (to a publication, for
example) would result in the uploaded file being cached twice (in
./public/uploads/tmp/...). This is because the file is cached when the
attachment model is instantiated and we were inadvertently instantiating
it twice: once in DocumentsController#build_document and then again in
DocumentsController::NationalApplicability#create. I added the failing
test and then fixed the double instantiation by overriding the
`#build_document` before filter so that it didn't fire for the `#create`
action.

app/controllers/admin/documents_controller/national_applicability.rb

@@ -2,6 +2,8 @@ module Admin::DocumentsController::NationalApplicability
   extend ActiveSupport::Concern
 
   included do
+    before_filter :build_document, only: [:new]
+
     before_filter :build_nation_inapplicabilities, only: [:new, :edit]
   end
 

test/support/admin_document_controller_test_helpers.rb

@@ -207,6 +207,18 @@ def should_allow_attachments_for(document_type)
         assert_equal greenpaper_pdf.size, attachment.file_size
       end
 
+      test "creating a document should result in a single instance of the uploaded file being cached" do
+        greenpaper_pdf = fixture_file_upload('greenpaper.pdf', 'application/pdf')
+        attributes = controller_attributes_for(document_type)
+        attributes[:document_attachments_attributes] = {
+          "0" => { attachment_attributes: attributes_for(:attachment, title: "attachment-title", file: greenpaper_pdf) }
+        }
+
+        Attachment.any_instance.expects(:file=).once
+
+        post :create, document: attributes
+      end
+
       test "creating a document with invalid data should still show attachment fields" do
         post :create, document: controller_attributes_for(document_type, title: "")