[LAD] Question about LV2 UI extension

I was looking at the (unfinished) example sampler plugin here: https://gitorious.org/gabrbedd/lv2-sampler-example I ran through the mental exercise of trying to figure out how to finish it, and I have a question about the UI extension. How does a UI tell its plugin to load a sample file? The example has a TODO in its run function that indicates it will react to an LV2_Event that contains a pathname for a file. I don't understand how a UI will create this event. On lv2plug.in, I see there's an experimental string-port extension that defines a transfer mechanism for strings. Is this the recommended method? Do any hosts support this extension? There's also an atom extension, but I don't think I grok it yet. Can I create a port of type atom:MessagePort? How does a UI make use of that?