Backup fails when REDMINE_ATTACHMENTS_DIR is changed from default

[jimgeorge]

In my setup, I had changed REDMINE_ATTACHMENTS_DIR to point to a different directory which I then used a bind mount to point to a different directory on the server. This causes the backup task (invoked with app:backup:create) to fail, since it expects the call to backup_dump_directory ${config_default_attachments_storage_path} .tar created a file that wasn't named files.tar, which is later referenced in the call to backup_create_archive. tar bombs out, since it can't find the file files.tar.