How to create a URL to an unmanaged public file in Drupal 8
You need to run the file path through file_create_url() and then use it together with base:

Url::fromUri(file_create_url($dest));


(file_create_url() should return absolute URL's, so you don't need base)

There's an issue to simplify that: https://www.drupal.org/node/2539622