If you're posting to usenet, you have to post the complete articles of your post. If you post only the NZB file, the articles to which the NZB refers (that's the only thing a NZB does), can't be found and as a result the post can't be downloaded.
ps. Regarding the upload speed, that depends on your provider. But usually the upload speed is much lower than your download speed, so it can be correct...