Protected content is only cached to RAM and not cached to the tmp folder.
Most others are cached there and can be retrieved. I do it all of the time.
If you are using FF I suggest you look into DownloadHelper. It is a much
easier solution. If you don't like that one there are several others that do
the same thing. I have not found a good one for Chrome or Chromium, though,
so go to the tmp folder for those.