How do I migrate the file attachments field from a content type?
In Drupal 6, file attachments are considered enabled if the upload-module is enabled, and the variable 'upload_[content_type]' is true or not set. The latter could be the cause that your attachment fields are not being migrated. 

There is an open bug report for this scenario, including a patch, at https://www.drupal.org/node/2829274