Jared... if the reason you asking for that file extension added is to share a 3D format file for a 3D printer... the recommended and standard method to share files is to ZIP the file up and attach that. Disk space and data bandwidth is a premium product with web servers and compressing the file down into a zip file saves on both.
Oh ok. The file is only 72kB so I thought it was not that big of a deal to just add it to the list of allowed file types that will simply upload like jpg, zip, etc...