As I understand it, the latest firmware and the latest version of Airport Utility took away the ability to hose your files over the internet. It can still be done, but only with third party software and, presumably, an always-on computer acting as a server (unless there is a workaround to re-enable the features on the Time Capsule). I haven't looked into the issue in depth though, I just recall reading something to that effect.
Why do you need this functionality? Perhaps there is a better way. I use DropBox (iCloud works the same way) for most of my files that I need to access elsewhere. Then, it's kept on secure enterprise level equipment, not my home router, and is thus much more secure. It's also much faster, as I'm not limited to the upload speed of my ISP.
Granted, this isn't the best solution if you have a lot of files or the files are big, but when working with big files, remember your home internet connection is likely only in the 1-3mbps upload range. That is VERY VERY slow when dealing with large files! A cloud solution is usually better, and if you can limit it in size, iCloud and Dropbox both have free options!