Refs #43 - ObjectDownloadView.get_file() raises FileNotFound if file field does not exists or is empty. Introduced DownloadMixin.file_not_found_response().
Refs #43 - ObjectDownloadView.get_file() raises FileNotFound if field is empty. Raises AttributeError if field does not exist (view or model is misconfigured).
Refs #43 - Updated CHANGELOG.
Merge pull request #64 from benoitbryon/43-file-not-found-404

Closes #43. ObjectDownloadView returns HTTP 404 if file field is empty