Migrate updates without using the --update or --needs-update option

Essentially, I am now out of luck with my file migration:

drush mi ImageAttachFileMigration --limit=10000
Processed 10000 (0 created, 10000 updated, 0 failed, 0 ignored) in 257.9 sec (2327/min) - done with 'ImageAttachFileMigration'                                                                                                    [completed]

Here is my code for reference:

  // File Migration for Image Attach Images
  $file_arguments = $common_arguments + array(
    'machine_name' => 'ImageAttachFileMigration',
    'description' => t('Import Drupal 6 File references to be used by content types that used image attach in D6.'),
    'user_migration' => 'UserMigration', // User migration must be run first so the ownership can be set.
    'default_uid' => 1,  // Seems appropriate
    'source_dir' => 'http://ranchomurieta.com/',  // See https://www.drupal.org/node/1540106
    //'destination_dir' => 'public://', // This is the default value.
  );
  Migration::registerMigration('DrupalFile6Migration', $file_arguments['machine_name'], $file_arguments);