Please stop writing your own HTTP parsers, consider Rack next time.

rack_files.rb

#!/usr/bin/env ruby

require 'rack'

FILES = {
  '/index.html' => 'index.html'
}

Rack::Handler.get('Thin').run proc { |env|
  path = env['PATH_INFO']

  if FILES[path]
    [200, {'Content-Type' => 'octet/stream'}, File.new(FILES[path])]
  else
    [400, {'Content-Type' => 'text/html'}, ['Fail']]
  end
}