How do I migrate an image attributes? I am migrating a Drupal 7 site to Drupal 8 using the Migrate API. So far, I have been able to do this without writing my own plugins.

How do I migrate additional information in an image field (e.g. the alt and title attributes)? 

So far I have been using this, but it does not seem to work.

field_bild:
  plugin: iterator
  source: field_bild
  process:
    target_id:
      plugin: migration
      migration: d7_file
      source: fid
      alt: alt
      title: title
Here is the format I found working:

field_teaser_image/target_id:
  plugin: migration
  migration: attachment_image
  source: file_id
field_teaser_image/alt: file_alt