Make sure your content-type is multipart/form-data.
curl -F "at=@/your/file/path" \ -F "c=yourComment" -F "api=" \ -H "Authorization:Basic YOUR_API_KEY_GOES_HERE" \ https://www.ragic.com/demo/ragicsales-order-management/10001/2
In the request body, the value of the parameter c is the comment(required), and the value of the parameter at is the attachment(optional).
Your API key could obtain here.