Thank you for getting back to me, Bill. I got it working!
I tried to remove the hardcoded File Extension on the object's External Data property and to use subdirectories for relative file locations. Got the same outcome as before.
But then I decided to double-check all the object target references on the actual Agentry screen definition itself - and therein lied the problem! My "Object Property" on the HTML control had a broken target reference. Once I pointed it to the correct object's External Data property everything worked perfectly. See the screenshots below:
Thanks again,
Roman.