As noted in Blacklight's answer, it's not recommended; but you can do it, and everything will work, just not optimally. Here's an explanation on why this might not be the best to do.
First, it makes sense Time Capsule wouldn't back up files stored directly on it. Where would it back them up—to itself, the same drive? That's not a backup of great value, but this isn't a true negative.
The bigger issue is that it's using a shared physical disk, with one potentially greedy process: Time Machine. It has nothing to do at all with network/wifi performance.
As you're trying to share the same disk, if Time Machine has many unpredictable reading/writing input/output occurring in the background, accessing other files on the same physical disk that Time Machine is accessing could result in sporadic performance hits. If Time Machine slows, no great worry, the user doesn't watch this background process. But if it takes too long to download/stream a large movie file because a heavy Time Machine write is simultaneously occurring, the user would think the performance of the Time Capsule was to blame. Apple doesn't want you to think its products are of poor quality, so they recommend you use them in ways that optimize perceived performance.
If you accept the possibility of slow access for stored items due to Time Machine commonly writing to the same disk, it should work fine (outside less space for the Time Machine image).
However, if you're expecting blazing and consistent access to content, don't use the same disk you're using Time Machine for anything else. Not even if you partition it. Get a separate drive, connect it to the Time Capsule, and use that. The second disk will not be affected by Time Machine's reading and writing processes.