curl --request POST \
  --url https://api.zapthinker.com/rag/upload/{instance} \
  --header 'Content-Type: multipart/form-data' \
  --header 'apikey: <api-key>' \
  --form 'files=<string>' \
  --form 'ragId=<string>' \
  --form files.items='@example-file'

{
  "success": true,
  "results": [
    {
      "filename": "<string>",
      "size": 123,
      "mimetype": "<string>",
      "success": true
    }
  ]
}