My Drupal 6 site has different content types, and there is a File Attachments field added to the content types. While migrating to Drupal 8.1, this field is not getting migrated, but the attached file is present in the files directory. While doing the migration, it is displayed that the "upload" module in Drupal 6 has migration path, yet it is not migrated.
How can I migrate the file attachment field to Drupal 8?
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
This is using the full upgrade process (/upgrade UI or drush migrate-upgrade command), I assume? Did the migration create an 'upload' field on the relevant content types (even if it didn't populate it)? The question is whether the failure is at this point (creating the field), or in connecting it to the actual files when migrating the nodes.
You might want to open a bug report under the "migration system" component: https://www.drupal.org/node/add/project-issue/drupal
