The %20 is the encoding for a space character. Spaces are illegal in HTTP fields, so a substitute must be found. The percent sign followed by a number can be used to insert an arbitrary character (and a space is 20).

As for the question of why the file was not found, I have no clue (maybe you moved it? Most Gnutella clients are too stupid to point the old index at the new location, even if it's the same f*in' file! ... try closing both Gnutella clients, and reopening them - which should cause a reindex and a cache drop ... see the rest of this paragraph for details). Files aren't actually served by name, though. They're served by index number (that number that comes before the file name in the GET request). You can even try downloading nonsense.mp3, and as long as you ask for a valid index number of a shared file, you'll actually download a file (which probably isn't really named nonsense.mp3, unless you're downloading from the famous SPAMmer who was cruising our network a couple of years ago - I wonder how many nonsense.mp3 files he's got lying around his system ... LOL).