This page refers to Models and then mentions FBX files and links to this page:
https://create.roblox.com/docs/parts/models
But this is incorrect and confusing. As far as I’m aware, the Assets API does not support uploading Roblox Model instances (which is what the linked page documents) or Roblox Model files (rbxm, rbxmx). But it does support uploading Meshes as FBX files. (It should support uploading rbxm!)
I’d suggest changing the row of the table to be labeled “Mesh” and link to https://create.roblox.com/docs/parts/meshes
Affected URL: https://create.roblox.com/docs/cloud/guides/usage-assets