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

Thanks @AstonJ !
At this moment it seems a bit complex for me but I felt the same with elixir Umbrella when started Elixir some time ago. I think once I will get more familiar with the basics I can go on the path suggested on the blog post. I just wanted to be sure there are such solutions before starting my new journey. ^^

Is there any particular reason why you want to do it?

Something I do with almost all of my Rails apps (thanks to a tip from Yehuda Katz) is to create a classes folder in my app directory and this is where all my non-Rails functionality lives - you may find this is sufficient