importing .x file

I was reading how the Axon Media Server can accept a .x file generated from a custom 3-d model.
However, when I tried to upload the file via the Content Manager, I received an error stating only .jpg, .m2v, etc... (with .x not being in the list) can be accepted.

Anybody have some advice regarding this issue?
Or, an alternative to loading my custom 3-D model.