Using sys.exit in a management command makes it impossible
to unit test the code in question. The correct approach to do the same
thing in Django management commands is to raise CommandError.
Followup of b570c0dafa
This commit adds the capability to keep track and remove uploaded
files. Unclaimed attachments are files that have been uploaded to the
server but are not referred in any messages. A management command to
remove old unclaimed files after a week is also included.
Tests for getting the file referred in messages are also included.