Enabling RPM installation without Developer Mode is a dangerous thing (as RPMs can brick your device and have root access), and I wonder how Filebrowser got through the QA with this feature at all. You should legally only be able to break your device after accepting the terms and conditions of Developer Mode. Anyway, you can install RPMs without Developer Mode with Cargo Dock, too, by exploiting a bug in the store-client (until it will be fixed). In another thread, somebody shortly described how to do it.