Fix broken cover photo for matrix-registration

roles/matrix-registration/tasks/init.yml

@@ -36,7 +36,7 @@
             Also fixing the form, which goes to /register.
           #}
           sub_filter_once off;
-          sub_filter_types text/html;
+          sub_filter_types text/html text/css;
           sub_filter "/static/" "{{ matrix_registration_public_endpoint }}/static/";
           sub_filter "/register" "{{ matrix_registration_public_endpoint }}/register";
         }