Refactor PublicUploadsController to remove helper

This helper is going to be factored out of all the attachment
controllers eventually.
Refactor AttachmentsController to remove helper

The helper is going to be removed in a future commit.
Remove unused UploadsControllerHelper
Show a 404 for unknown files

This has the side effect of showing a 404 to unauthenticated users as
well, which is probably desirable (as to them the file is functionally
non-existent).

This also involved removing an old test that doesn't apply any more.
Merge pull request #878 from alphagov/fix-uploaded-attachment-placeholder

Fix uploaded attachment placeholder page

https://www.pivotaltracker.com/story/show/55919706
https://www.pivotaltracker.com/story/show/55222932