Open 0xr.space or the Model Explorer app, press Open and choose the file as it is, even if it is called download with nothing after it. Before refusing a file without a known extension, Model Explorer reads its first 4 KB and recognises the format from what is inside: GLB and glTF, FBX, STL, OBJ, PLY, USDZ, USDC and USDA, 3MF, .blend, .3dm, COLLADA, VRML, STEP, DXF, LAS point clouds, G-code, splat files, and ZIP, RAR or 7z archives. The file is given the right extension and opens as usual. Nothing is uploaded to do this.
Some files cannot be told apart this way. An OBJ is recognised only when its first lines are geometry, a binary STL only when its size matches its triangle count, and a glTF's .bin or an OBJ's .mtl has no signature of its own. When nothing matches, the message ends The chosen file has no extension and its contents are not a format we read.
The headset's file picker, or the site it came from, can drop the name and leave a file called download. The contents are still the model.
No. The first bytes are read on the device, and nothing is sent to a server unless you create a share link yourself.
The .gltf itself is recognised, but its .bin and images need their own names to be matched to it. Choose a ZIP of the whole folder instead.