curl --request POST \
  --url https://your-instance.example.com/api/messages/upload-attachment \
  --header 'Content-Type: multipart/form-data' \
  --form file='@example-file' \
  --form 'webchatConfigId=<string>'

{
  "success": true,
  "attachment": {
    "storageKey": "<string>",
    "filename": "<string>",
    "contentType": "<string>",
    "size": 123
  }
}