Fix file manager helper

With the changes in #1367 the file manager helper always returns a value, which created a bug with the file manager helper when validating form submissions. If you submit a form on a single page with a file field that's empty and form validation fails, when the page renders the validation errors the file helper will try to create a file object from the post value, which is 0. It doesn't check if it's an object, so there's an error when it checks the isError() method on null. This pull fixes this by checking if the file is an object.