Well, regarding file uploads you can use shrine which is really well desgined and have a thin wrapper for hanami (https://github.com/katafrakt/hanami-shrine)
For authentication there is the awesome (as any thing by jeremy evans) rodauth .
This blog post shows a bit of its capabilities.
Unfortunately I cant really point you to any admin panel.
Rails is still awesome for a quick and dirty mvp, but I have been bitten so many times by devise, or autoloading issues, or active support black magic that I prefer to have a bit slower start using something “simpler” like hanami or even go straight to sinatra/roda/hanami-router