edit uploaded file/image

I upload a file with the following:
  @post = Post.first()
  result = Post.upload(uploaded_file.tempfile, uploaded_file.original_filename, content_type: uploaded_file.content_type)
  @post.thumbnail = {"name" => result["name"], "__type" => "File", "url" => result["url"]}

When I want to edit this file I use the same code but I get the following error:
  IOError at /posts
  closed stream
