Rails上载的文件为空

Processing Admin::HeadlinesController#update (for 127.0.0.1 at 2010-03-08 12:26:13) [PUT]
  Parameters: {"commit"=>"Save changes", "action"=>"update", "_method"=>"put", "authenticity_token"=>"mK70XRk5gOPUwXOcNboT/4K8PD9RBM7GqCOlEUKZwcA=", "headline"=>{"position"=>"1", "location"=>"primary", "attachment_id"=>"13", "headline_content"=>"questionnaires", "article_id"=>"3", "image"=>#<File:/tmp/RackMultipart20100308-63211-1vym9nj-0>}, "id"=>"140", "controller"=>"admin/headlines"}

但如果我看一下 /tmp/RackMultipart20100308-63211-1vym9nj-0 文件为空。我认为这应该是我上传的文件对吗?