It takes time for all of the different news servers to get your post, but they will get it. If you didn't want everyone to see your reply then you should just use e-mail to send you a reply. Because of how the Internet news system works your message may not appear for a little while, but it has been posted unless you have gotten an error message back from your newsreader. Your message will arrive at different times on different news servers across the Internet, but it will make it to all the ones that carry the news group you are posting to eventually. If you are using a newsreader other than Netscape then it should have a choice that allows you to download binary files.
Most good newsreaders support downloading files.
They will do it automatically and put together the differ pieces of the post. If you are downloading a file that has been encoded you may need another piece of software to decode the file after it has been downloaded. Netscape will download picture so that you can see them as they are download, but it doesn't handle multi-part files yet. For more information you should go to the different OS FAQ's. Though we won't know how to do this in your newsreader, you should have a choice that allows you to attach a file on a post.