deflator.rb

class Deflater
  def initialize app
    @app = app
  end

  def call env
    if acceptable_method?(env) && compress?(env)
      Rack::Deflater.new(app).call env
    else
      app.call env
    end
  end

  private

  attr_reader :app

  def acceptable_method? env
    ['GET', 'HEAD'].include? env['REQUEST_METHOD']
  end

  def compress? env
    env['REQUEST_PATH'] !~ /\.(png|jpg|jpeg|gif)\z/
  end
end

production.rb

  # Sets the correct headers when serving files from /public. CloudFront
  # will forward these headers to the client.
  config.middleware.insert 0, 'Rack::Static',
    urls: Rails.root.join('public').children(false).map { |p| "/#{p}" },
    root: Rails.root.join('public'),
    header_rules: [
      # Clients should cache assets for one year.
      ['/assets', {'Cache-Control' => 'public, max-age=3136000'}],

      # Provide web fonts with cross-origin access-control-headers
      # Firefox requires this when serving assets using a CDN.
      [:fonts, {'Access-Control-Allow-Origin' => '*'}]
    ]