flash_session_cookie_middleware.rb

require 'rack/utils'

class FlashSessionCookieMiddleware
  def initialize(app, session_key = '_session_id')
    @app = app
    @session_key = session_key
  end

  def call(env)
    if env['HTTP_USER_AGENT'] =~ /^(Adobe|Shockwave) Flash/
      params = Rack::Request.new(env).params
      unless params[@session_key].nil?
        env['HTTP_COOKIE'] = "#{@session_key}=#{params[@session_key]}".freeze
      end
    end

    @app.call(env)
  end
end

flash_session_middleware.rb

Rails.application.config.middleware.insert_before( ActionDispatch::Session::CookieStore, FlashSessionCookieMiddleware, "_xyz")

## Where "_xyz" is yours Session Cookie name

gistfile1.mkd

== Using Rails3 with uploadify

Add this to 'scriptData' in js (which is loading uploadify)

  '_maxhost_upload': $('meta[name=cses]').attr('content')

And this to yours application layout

    %meta{ :name => "cses", :content => cookies["_xyz"]}

where _xyz is Yours session cookie key.

gistfile4.txt