>>401178819
Assetbundles are simply the best choice for dynamic user-content. They support loadable scenes (i.e. worlds), and loadable assets (prefabs, anything else). They can even be made to include and load custom scripts with 1 extra line of code. Unity is pants on head retarded for not embracing this explicitly as a feature for custom content.
I don't think Bonelab is going to use uploading for custom content though. Unless there's workshop support, it will probably just use a local folder to drop files into. There is no multiplayer, so no need to host the files anywhere.