`KeyError: 'binary/octet-stream'` during MM2.1 import

During importing quite a huge MM2.1 mbox using the --since option as recommended by @msapiro in this post the following error occurred after some minutes:

# docker-compose exec mailman-web ./manage.py hyperkitty_import -l listname@domain --since 2014/07/01 /import/listname.mbox
Importing from mbox file /import/listname.mbox to listname@domain
\Traceback (most recent call last):
  File "./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python3.6/site-packages/django/core/management/__init__.py", line 371, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python3.6/site-packages/django/core/management/__init__.py", line 365, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python3.6/site-packages/django/core/management/base.py", line 288, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/lib/python3.6/site-packages/django/core/management/base.py", line 335, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python3.6/site-packages/hyperkitty/management/commands/hyperkitty_import.py", line 283, in handle
    importer.from_mbox(mbfile)
  File "/usr/local/lib/python3.6/site-packages/hyperkitty/management/commands/hyperkitty_import.py", line 156, in from_mbox
    add_to_list(self.list_address, message)
  File "/usr/local/lib/python3.6/site-packages/hyperkitty/lib/incoming.py", line 121, in add_to_list
    email.content, attachments = scrubber.scrub()
  File "/usr/local/lib/python3.6/site-packages/django_mailman3/lib/scrub.py", line 77, in scrub
    attachments = self._get_all_attachments()
  File "/usr/local/lib/python3.6/site-packages/django_mailman3/lib/scrub.py", line 104, in _get_all_attachments
    attachments.append(self._parse_attachment(part, part_num))
  File "/usr/local/lib/python3.6/site-packages/django_mailman3/lib/scrub.py", line 156, in _parse_attachment
    payload = part.get_content()
  File "/usr/local/lib/python3.6/email/message.py", line 1087, in get_content
    return content_manager.get_content(self, *args, **kw)
  File "/usr/local/lib/python3.6/email/contentmanager.py", line 25, in get_content
    raise KeyError(content_type)
KeyError: 'binary/octet-stream'

The mails of the first 3 years got correctly imported, last year is missing.