Many clients (BearShare, Gtk-Gnutella, LimeWire) emit a Content-Disposition header when you download a file, so if you have a working source, that would tell you one of the used filenames. You have to be a bit cautious with the encoding for non-ASCII filenames though; LimeWire and Gtk-Gnutella should always give you UTF-8 encoded and NFC normalized filenames. BearShare probably doesn't. They also use a slightly different format, for example, Gtk-Gnutella explicitely declares the filename being UTF-8 encoded, the others don't. You'll see the effect of this when you're trying to download something with Mozilla - well LimeWire and BearShare probably block browsers anyway but you get the idea.