Restrict the listed submission files for the current user.

def get_queryset(self, request):
        ''' Restrict the listed submission files for the current user.'''
        qs = super(SubmissionFileAdmin, self).get_queryset(request)
        if request.user.is_superuser:
            return qs
        else:
            return qs.filter(Q(submissions__assignment__course__tutors__pk=request.user.pk) | Q(submissions__assignment__course__owner=request.user)).distinct()