file_transfer() sets headers before checking file existence

Pretty sure 8.x doesn't suffer from this, but 7.x does.

file_transfer() tries to issue a drupal_not_found() if the file doesn't exist, however before it does that, it sets http headers. In Drupal 6 image module at least, this includes Content-type image/png or whatever. When you send a Content-type header like that, and then try to print a normal Drupal 404 page, at least Varnish doesn't like this and spits out a 503, not very pretty.