Rewriting the output of a file download field
The "File download fomatter" you're using doesn't output just the path to the file, it's a more complex formatter and you can't use it in your case as a path to download the file. 

In Drupal 8 there is a separate Field in File Views called URI that you need to use. 



Depending on which Drupal 8 version you're using you'll see only the formatter File URI and you need to tick the option Display the file download URI to get the link you can use in an anchor href attribute later when overriding fields. 

Do not enable Link this field to the file download URL, this outputs complex anchor HTML, not just the path that you're after. 

You can exclude it from display, position it above the image field and Rewrite results for the image: 

&lt;a href="{{ uri }}"&gt;{{ image_field }}&lt;/a&gt;

or let Drupal link it: