File attachments via "post /checklists/checklist_id/import.(json|xml)"?

In the api docs at Open API | Checkvist, “Create list items”, the POST /checklists/checklist_id/import.(json|xml) endpoint has an add_files[1] key which it seems can be used to import file attachments via the API.

What’s the format for the attachment data (assuming I’ve read correctly that this is possible)? I examined some exports, thinking they might model the format to follow, but they seem to have urls or refs to existing files (S3 objects?), depending on the export format.

Can files be attached base64 encoded, or perhaps via url?