I just tried uploading a 150MB file 10 times to a php app behind a nginx proxy and it worked flawlessly every-time. I am using latest chrome and I have a 10 Mbps upload speed in bay area.
So I think we might need to revise our assumption that in real world 100+ MB file upload with http is fragile.
I love discourse the way it is. Its a great product and you as a co-founder have made a lot of good decisions to get it where it is.
I am trying to present a data point in the hope that you revise your current assumption that 100+ MB file upload over HTTP is fragile. 