curl --request GET \
  --url https://api.stackone.com/unified/messaging/messages/{id}/attachments/{subResourceId}/download \
  --header 'Authorization: Basic <encoded-value>' \
  --header 'x-account-id: <x-account-id>'

{
  "headers": {
    "content-disposition": "attachment; filename=\"example.pdf\"",
    "content-type": "application/pdf",
    "content-length": 1024,
    "content-range": "bytes 0-1023/2048",
    "content-encoding": "gzip",
    "transfer-encoding": "chunked"
  },
  "data": "<string>"
}