Hi KooHyun,
The ListedObject entity is a specialization/subclass of the S3Object entity/superclass. What this means is that it gains the characteristics that the parent class has assigned to it, in this case the Key attribute is the one that you're looking for.
I've created a sample microflow that would log an S3 object's Key attribute in the Mendix Studio Pro console down below:
I hope this helps!