[whatsapp] make DocumentMessage handler use FileName attribute.

referenced: https://github.com/Rhymen/go-whatsapp/blob/master/message.go#L582

bridge/whatsapp/handlers.go

@@ -359,7 +359,7 @@ func (b *Bwhatsapp) HandleDocumentMessage(message whatsapp.DocumentMessage) {
 		return
 	}
 
-	filename := fmt.Sprintf("%v%v", message.Info.Id, fileExt[0])
+	filename := fmt.Sprintf("%v", message.FileName)
 
 	b.Log.Debugf("Trying to download %s with type %s", filename, message.Type)