file_controller2.rb

# files_controller.rb
class FilesController < ApplicationController
  def index
    @files = Files.all
  end
  
  ...
end

files_controller.rb

# posts/files_controller.rb
class Posts::FilesController < ApplicationController
  def index
    @posts = Post.find(params[:post_id])
    @files = @post.files
  end
  
  ...
end

routes.rb

resources :posts do
  resources :files, controller: 'posts/files'
end

resources :files

this way you prevent some conditionals are made to happen your action and keeps the controller most organized