1. Export from Blender as "gLTF separate" ( https://i.gyazo.com/353470ee8a3bb81f7f290f272372d061.png ), give it a filename with a space in it (e.g. "My Mesh")
  2. Try to upload, get https://i.gyazo.com/654fb82d7ff068a5f6aba2a1ff2a37b3.png
If you open the .gltf file in a text editor you will find: https://i.gyazo.com/f65665874e923e41314103861103ebe6.png
If the %20 is replaced with a space in the file, the GLTF uploads successfully.
Conclusion: either Blender is doing wrong by making the space %20 in the .gltf file, or the uploader is not doing what it should to handle the URL encoding.
This problem exists in both SLV (info attached) and the current FS Beta (7.2.2.79281).