@Kovid: I've found out more about the Kobo browser and why it isn't working with the new server.
Basically, the browser is getting the file name from the URL. If it can't find one, or the file name isn't one it can handle, the file isn't downloaded. The browser doesn't use the "Content-Disposition" header field, so it cannot download the file. I've put a longer description at
https://www.mobileread.com/forums/sh....php?p=3531644. I have reported the problem to Kobo. We'll see when a fix comes out.
I've hacked some code to include a file name in the requests to prove it works. That of course broke other things, but the file downloaded.