I've got ImproperlyConfigured error after attaching extra view to admin class

Single extra view class CANNOT be reused in multiple admin sites. To use it in multiple admin sites create separated class of extra view:

class MyView(RalphDetailViewAdmin):
    icon = 'chain'

class MyView2(MyView):

Then use it in your admin:

class MyAdmin(RalphAdmin):
    change_views = [MyView]

class MyAdmin2(RalphAdmin):
    change_views = [MyView2]

Attachments app is an example usage of this mechanism - for every admin site that uses AttachmentsMixin, there is created separated class of AttachmentsView.