There is one other file type using
the X3D file extension!
.x3d - X3D graphics model
X3D file extension - 3D crossword plugin for Netscape/IE/Opera
What is x3d file? How to open x3d files?
File type specification:
The X3D file extension is used for a 3D crossword plugin for Netscape/IE/Opera developed by Horizon, A Glimpse of Tomorrow Inc.
MIME: application/vnd.hzn-3d-crossword