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 DrupalfilePluginmigrateprocessd6FieldFile and ran the migration with the settings below I got the correctly referenced files and the alt/title text.
// Configure the migration process plugin to look up migrated IDs from // a d6 file migration. $migration_plugin_configuration = $configuration + [ - 'migration' => 'd6_file', + 'migration' => 'upgrade_d6_file', 'source' => ['fid'], ];
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