Preview files outside Public or Assets folder in Rails

files_controller.rb

# In your controller use the Rails in-built function, send_file to send the file to your browser for preview. 
# Specify the filepath of your file in the send_file function.

def preview
    send_file "#{Rails.root}/uploads/pictures/#{@file.name}"
end

routes.rb

# Create a route for your preview function. 

resources :files do
    member do
      get 'preview'
    end
end

show.html.erb

<!-- In your view page, use the various tags provided by Rails to display your file.-->

<%= image_tag preview_myfile_path(@file) %>