curl --request POST \
  --url https://api.example.com/v2/knowledge-bases/{kb_id}/documents/files \
  --header 'Content-Type: multipart/form-data' \
  --form 'files=<string>' \
  --form 'document_languages=<string>' \
  --form files.items='@example-file'

{
  "processing_status": {
    "status": "processing",
    "message": "<string>",
    "document_ids": []
  }
}