上传图片时出错

```
Started POST "/photos" for 127.0.0.1 at 2013-07-24 14:11:42 +0800
Processing by PhotoController#create as JSON
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"eZSHBs7WG+Fa4m8s1RlyD5Tqj6jfdDWz+HUiQv/jWlk=", "event"=>{"title"=>"", "slug"=>"", "compound_start_time_attributes"=>{"date"=>"", "time"=>""}, "compound_end_time_attributes"=>{"date"=>"", "time"=>""}, "location"=>"", "location_guide"=>"", "content"=>""}, "files"=>[#<ActionDispatch::Http::UploadedFile:0x007f9c97939910 @original_filename="QQ20130711-1.png", @content_type="image/png", @headers="Content-Disposition: form-data; name=\"files[]\"; filename=\"QQ20130711-1.png\"\r\nContent-Type: image/png\r\n", @tempfile=#<Tempfile:/var/folders/vp/86c14m092hq5_ywqlnbprffw0000gn/T/RackMultipart20130724-85909-somqgh>>]}
  User Load (0.5ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 4 LIMIT 1
   (0.1ms)  BEGIN
   (0.3ms)  ROLLBACK
Completed 500 Internal Server Error in 49ms

Errno::ENOENT (No such file or directory - identify):
  app/controllers/photo_controller.rb:6:in `block in create'
  app/controllers/photo_controller.rb:5:in `map'
  app/controllers/photo_controller.rb:5:in `create'
  app/middlewares/fallback_url_redirector.rb:9:in `call'


  Rendered /Users/mikelo/.rvm/gems/ruby-2.0.0-p195/gems/actionpack-3.2.13/lib/action_dispatch/middleware/templates/rescues/_trace.erb (1.2ms)
  Rendered /Users/mikelo/.rvm/gems/ruby-2.0.0-p195/gems/actionpack-3.2.13/lib/action_dispatch/middleware/templates/rescues/_request_and_response.erb (1.1ms)
  Rendered /Users/mikelo/.rvm/gems/ruby-2.0.0-p195/gems/actionpack-3.2.13/lib/action_dispatch/middleware/templates/rescues/diagnostics.erb within rescues/layout (10.2ms)

```

* OSX 10.8
* ruby2.0(rvm)