Question: I'm new in Ruby/Rails, so, my question: what is the perfect way to upload file and save filename in database with rails? Are there any Gem? Or maybe there are good build-in feature?

Answer: 
You can try Paperclip.It is most popular gem in rails community...

```
   https://github.com/thoughtbot/paperclip

```

these lines do the all stuff

```
   class User < ActiveRecord::Base
     has_attached_file :avatar, :styles => { :medium => "300x300>", :thumb => "100x100>" }
   end

```

Try it......
Most people use the [Paperclip Gem](https://github.com/thoughtbot/paperclip).