Hello Kalle,
Wordpress uses MIME checking for uploaded files, so you need to extend this function in order to be able to upload GLB or GLTF files.
There are several ways how to do it. One of them is implemented in my plugin for Wordpress - Babylon.js 3D Wordpress Plugin .
It is much more convenient to have all your models at the same site than to have external location for 3D files!
Feel free to ask any questions!