assets_controller.rb

require 'mime/types'

class AssetsController < ApplicationController
  session :cookie_only => false, :only => :create

  def create
    @asset = Asset.new(params[:asset])

    respond_to do |wants|
      wants.html do
        if @asset.save
          redirect_to asset_url(@asset)
        else
          render :action => :new
        end
      end

      wants.json do
        @asset.content_type = MIME::Types.type_for(@asset.filename).to_s
        if @asset.save
          render :json => { :result => 'success', :asset => @asset.id }
        else
          render :json => { :result => 'error', :error => @asset.errors.full_messages.to_sentence }
        end
      end
    end
  end

end