Images are not migrated and the body field is empty
The solution for the body-field:

it must be a translation problem. I changed the language of the Drupal 8.1 site to the same language as the Drupal 7 site and now the body field isn't empty anymore.

But the images are still missing. I noticed that no files were imported. Under /admin/content/files there is nothing! any help in this case?