What it opens
GLB · GLTF
Full materials
PBR channels, embedded textures, scene hierarchy and node names all come through intact.
OBJ · FBX
Materials where present
Keep the MTL file or the texture folder next to the model and drop the whole lot in together.
STL · PLY · 3MF
Geometry, shaded neutrally
These formats hold no material data, so what you see is the form alone — that is not a broken file.
Viewer questions
Does the file get uploaded?
No. The viewer reads the file straight from your disk into the browser. Nothing is transmitted, which is also why it works with no account and no limit.
Can I share a link to what I am looking at?
Not for a local file, for the reason above. Assets on your shelf can be given a view-only link from the spec sheet.
Why does my model look grey?
Either it is an STL or PLY, which carry no materials at all, or it is an OBJ whose MTL and texture files were left behind. Drop the whole folder in rather than the single file.
The converter, the viewer and the Gridfinity generator are all free and all run in this tab. See all three.