from first look, the rule looks ok. Please, take into account that this will produce one long line combining all lines in given text file.
You don't required to use placeholders, until you have a reson for it - for example, if files are called according to user or group names, or some other variable parameters, so you can avoid creation of separate configuration for every file on your disk. If no placeholders were specified in the External Lists settings, then specification of property parameters in the rule will have no effect.
If you have more questions on external lists, then you can ask them here.