Not necessarily changes the filename, but you end up with a file of same file-hash (identical file content) but with another name, perhaps the original name. So the original file chosen to download was probably a masked file with a less suspicious file name. Once you've been using gnutella network for quite a while you will notice files you download sometimes have a different filename to the one you actually chose to download. Perhaps this is still good because you end up with what you want, else you end up with something else because it was a poor choice in the beginning.

As old as the post is, Leeware's post here is quite relevant now: Quality of Content on the Network

If you wish to filter it all out ... or much of it, see Direct instructions to eliminate porn & other marketing junk from results