I used plsh extension but I had to use two functions and a trigger, see code below
I don't see any way to not use a trigger given your requirement. And as written I don't see that you need the second function - just write the trigger function in plsh. Admittedly its probably better to have two functions from an architecture standpoint.
Listen/Notify doesn't seem to make sense as you want the server to ensure that the file exists - not some other client that may or may not be connected.