Add real responses to the /mapconfig/list[audio|video|image] endpoints

[jacobwod]

When NodeJS backend was created the endpoints specified in the title were only added as stubs, always responding with empty arrays. The reasoning behind that decision was that since Admin UI lacks the possibility to actually upload files, the solution should be seen as temporary and will be replaced in a near future with a more robust solution.

That didn't happen and we're still here, 3 years later.

So I'm adding those endpoints to confirm with the API specification. I'll be adding some nice settings too, such as the ability to actually configure which directory holds the uploaded files (since App_Data/Upload might not always be the most obvious location). I'll also add the option to specify which extensions are considered valid for each of the three types.