How to Make Managed File Field Available to Twig Template?
Since my rep isn't high enough to comment... yet, I have to add my comment as an answer.

My initial thought is that the file would be saved as an entity.  After reading up a bit on some d.o comments, there is/was a bug where managed_files are temporary by default.  Somewhere in the mix you'll need to add something similar to 

   $image = $form_state-&gt;getValue('image');
   $file = \Drupal\file\Entity\File::load( $image[0] );
   $file-&gt;setPermanent();
   $file-&gt;save();


to make the file permanent.
(credit to This question)

Most likely you will need to save the file itself as an entity in the blockSubmit function.

    $file = \Drupal\file\Entity\File::load($image_id[0]);
    // Change status to permanent.
    if(gettype($file) == 'object'){
      $file-&gt;setPermanent();    
      // Save.
      $file-&gt;save();
      $user = \Drupal\user\Entity\User::load(\Drupal::currentUser()-&gt;id());
      $file-&gt;setOwner($user);
    } ... and so on.


After that, you should be able to pass the filename or file entity to your theme layer and load it in your preferred method.

Examples modified from This d.o post