'if' tests the other way around

[awhileback]

The conditional check on attachments being disabled in the config is broken (on Django 3.1, haven't tested earlier versions). It tests the other way around from the way it currently works, this fixes it so people who have attachments disabled no longer have to specify other attachment settings.

[amrishparmar]

I think the existing behaviour is correct here. The urls should only be getting appended if attachments are enabled or equivalently not disabled. The proposed change would append the urlpatterns even if the feature is disabled.