Fix import and export of page block file references

Exporting and importing of page block file references is broken.

When exporting a site, the file or image references are not included in the exported yaml even though the file itself is exported.

Sorry for the lack of tests at the moment, I am unsure which the is appropriate place(s) to included them as it is affecting export and import for both `./test/lib/fixtures/page_test.rb` and `./test/lib/fixtures/file_test.rb`.

I'll be happy to add the tests, hopefully this starts a conversation though.