Image downloads not being restricted

Image downloads are not being restricted by node permissions when private download method is set in drupal.

Its possible to download images by linking directly to them, even when user has no permissions to view node/attached images.
Also, since the image module bypasses upload method on download, all uploaded image files outside the path of image module would also be compromised.

Patch attached addresses these issues. Aditional content name and lenght headers are also being set.