ME: added 'small_download_url' property in attachment serializer and documentation for it
ME: fixed failing teset in test_attachment_viewset.py
ME: refactored attachment test
ME: updated attachment documentation
ME: added 'medium_download_url' property
ME: refactored 'retrieve' method to include 'export_handler'
ME: added list action
ME: added a test for data endpoint xls export
ME: added ability to access list action without 'pk' or when pk is 'public'