Migrate Alt and Title text from a D6 image file field to D8
So this worked for me not sure why. But my file migration id was upgrade_d6_file and when I change this in line 54 of Drupal\file\Plugin\migrate\process\d6\FieldFile and ran the migration with the settings below I got the correctly referenced files and the alt/title text. 

Diff

  // Configure the migration process plugin to look up migrated IDs from
  // a d6 file migration.
  $migration_plugin_configuration = $configuration + [
-   'migration' =&gt; 'd6_file',
+   'migration' =&gt; 'upgrade_d6_file',
    'source' =&gt; ['fid'],
  ];


Migrate yml

field_primary_image:
    plugin: d6_field_file
    source: field_image
    process:
      target_id:
        plugin: migration_lookup
        migration: upgrade_d6_file
        source: fid
      alt: alt
      title: title


Or even better is write your own process plugin like this: https://github.com/mikeryan776/d8-migrate-example-001/blob/master/migrate_carlyle/src/Plugin/migrate/process/CarlyleCckFile.php