The file api.py doesn't exist. The URL's you're looking at, for the most part, are virtual.

The Poser application server is not a web server. It's an application server. That means that strings map to procedures, not files. Hitting one of them invokes a procedure. Those procedures may, in turn, cough up a file, or may do a calculation.

As I said, even if there is an assets folder, I do not think the assets necessarily come from there. The only images I see in my installation are the category icons, and those are not being served to the GUI. The ones in the GUI are yellow when you select them. 

The GUI icons are layers in a PSD file, and are extracted by a procedure associated with the asset URL.

Similarly, the api.py is associated with a procedure and has nothing to do with an api.py file. In early development, I prototyped the server as an actual Python script, and the URL remained as an artifact of that early form of server. But the production server is embedded in Poser, not some external script file.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)