How to migrate fields attached to files?
You need to specify the bundle type when registering the dynamic migration:

  $image_args = array(
    'machine_name' =&gt; 'images',
    'class_name' =&gt; 'MyModuleImageMigration',
    'description' =&gt; t('Migrate images'),
    'user_migration' =&gt; 'users',
    'default_uid' =&gt; 1,
    'source_dir' =&gt; 'http://example.com/sites/default/files',
    'destination_dir' =&gt; 'public://',
    'skip_pictures' =&gt; TRUE,
    'bundle' =&gt; 'image', // Need to add the bundle type
  );
  Migration::registerMigration('MyModuleImageMigration', 'images', $image_args + $common_args);