d6_field_instance_widget_settings seems to fail with filefield_widgets

Does this help?

So the weird thing here is that the widget type migration is already specifically asserted in MigrateFieldWidgetSettingsTest. So I'm not sure why it's failing for you -- but the fact that it worked when the mapping was explicit in the migration entity versus when it was (supposed to be) put there dynamically by the migration builder is very telling. I don't know if I can reproduce this without looking at your specific case.

But I do have a theory -- did you have the Drupal 8 File module enabled when you ran the migration? It contains the cckfield plugin which sets up the correct mapping, so if you didn't have the module enabled, this could very well be the source of the failure.

Added a test, but it passes against 8.0.x. Not sure how to reproduce your issue.

@neclimdul and I did a screen share and figured out why this was happening -- it's because the migrate-manifest command is not aware of the builder system, which must be invoked in order for the proper widget mapping to be added to the migration.

Attached patch adds builder support to migrate-manifest.

Sorry for reopening this issue, I have the same problem and it seems that I am not alone, see: #2767237: Files not attached to nodes using drush migrate-import. The attaching in that issue works with additional settings in #13, but the filefield is not migrated (correctly).

According to #2 I added to my customized d6-to-d8 migration (in modules/custom/my_migration/config/install/migrate_plus.migration.upgrade_d6_field_instance_widget_settings.yml the filefield_widget mapping filefield_widget: file_generic:

process:
  # ...  
  options/type:
    type:
      # ...
      map:
        # ...
        filefield_widget: file_generic

I uninstalled Migrate manifest module, as I am not using it anymore, but anyway, the problem persists. Not sure if it is a core bug or migrate plus issue or a problem with my setup.

Maybe related to this issue is, that the field formatters aren't set for images/files. In migration messages I can find for example:

Failed to lookup field type array ( 0 => 'filefield', 1 => 'image_plain', ) in the static map.
Failed to lookup field type array ( 0 => 'filefield', 1 => 'default', ) in the static map.
Failed to lookup field type array ( 0 => 'filefield', 1 => 'imagefield__lightshow2__img_in_page__original', ) in the static map.
Failed to lookup field type array ( 0 => 'filefield', 1 => 'Thumbnail_linked', ) in the static map.

And the fields are set to hidden.

Hard to imagine what the fieldfield_widget error would have to do with migrate_plus - since that's raised in #2767237: Files not attached to nodes using drush migrate-import, that's a better place for further diagnosis (although still more likely to do with core than migrate_upgrade).