curl --request POST \
  --url https://scx-sbx.api.jtl-software.com/v1/seller/ticket/attachment/{channel} \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: multipart/form-data' \
  --form 'attachmentData={
  "sellerId": "4711",
  "ticketId": "CASE-01071982",
  "filename": "product.png"
}' \
  --form attachment='@example-file'

{
  "errorList": [
    {
      "code": "VAL100",
      "message": "Required field sellerId not found",
      "severity": "error",
      "hint": "Check the field `sellerId` — it must be a non-empty string."
    }
  ]
}