The Rack::TryStatic middleware delegates requests to Rack::Static middleware trying to match a static file

config.ru

# config.ru

# The Rack::TryStatic middleware delegates requests to Rack::Static middleware
# trying to match a static file
#
# Inspired by original code from rack-contrib
# http://github.com/rack/rack-contrib/blob/master/lib/rack/contrib/try_static.rb
#
module Rack

  class TryStatic

    def initialize(app, options)
      @app = app
      @try = ['', *options.delete(:try)]
      @static = ::Rack::Static.new(lambda { [404, {}, []] }, options)
    end

    def call(env)
      # make your life easier
      req = ::Rack::Request.new env
      
      valid_response = nil
      
      # try to load file from its original path or including suffix
      @try.each do |suffix|
        response = @static.call(env.merge('PATH_INFO' => req.path_info + suffix))
        
        # try it until there's a valid response
        break if response[0] != 404 && valid_response = response
      end
      
      if !valid_response &&                      # there's no valid response
          req.get? &&                            # it's a GET request
          !req.referer &&                        # there's no referer
          !req.path_info.end_with?('/') &&       # it's not a DIR path
          req.query_string.empty? &&             # there's no query string
          ::File.extname(req.path_info).empty?   # it's not a link to some file
          
        # actual file path is wrong, and it looks like a DIR path, so redirect it
        valid_response = [301, { 'Location' => ::File.join(req.url, '/') }, []]
      end

      valid_response or @app.call(env)
    end
    
  end
  
end

use Rack::TryStatic, :root => 'public', :urls => %w[/], :try => ['index.html']

# run your own Rack app here or use this one to serve 404 messages:
run lambda { |env| [404, { 'Content-Type'  => 'text/html' }, ['404 - page not found']] }