@griffinboy After building Rhapsody I went off the idea. Managing one site's downloads is enough work for me. If I was doing it as a commercial product then it might be worth my time, but then I'd probably have to make it closed source which I'm not going to do.
@Christoph-Hart It seems that the return type for the download callback is not working properly. I can download a file from my server, but the info parameters is always undefined, so you can't check the progress or completion.