I have experience this problem also. On my site, people want to share policies and research in a pdf format. The link is like a ning api file up load link. I also experience this when sharing links with the ning shortner. These are the only two issues that exposes my network as a ning network.
You can always open an Amazon Cloud storage account. I think a gigabyte of transfers a month will cost like .14 cents. In the dns of your registrar, you create a subdomain of something like downloads.yoursite.com. You then create a "Bucket" in Amazon with that same name and point your DNS (For the sub domain you created) to that bucket.
Then that download will look like it is being downloaded from downloads.yoursite.com
Thank you for your response.