backups: PermissionError when trying to download an exported backup archive.

Debian testing Plinth 0.40.0 amd64 FreedomBox manually installed on a Debian cloud instance

PermissionError: [Errno 13] Permission denied: '/var/lib/freedombox/FreedomBox-backups/FreedomBox_backup_2018-10-150929.tar.gz'
[2018-10-15 09:37:22,666] django.request ERROR    Internal Server Error: /plinth/sys/backups/download/Root%20Filesystem/FreedomBox_backup_2018-10-150929.tar.gz/
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "/usr/lib/python3/dist-packages/django/core/handlers/base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/lib/python3/dist-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python3/dist-packages/django/views/generic/base.py", line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/django/views/generic/base.py", line 88, in dispatch
    return handler(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/plinth/modules/backups/views.py", line 124, in get
    with open(filepath, 'rb') as file_handle:
PermissionError: [Errno 13] Permission denied: '/var/lib/freedombox/FreedomBox-backups/FreedomBox_backup_2018-10-150929.tar.gz'