pbox, we use a proprietary file format for 3D models. It contains the raw models (engine) data with some optimizations that occur at the time of export. At the moment, there is no any publicly available exporter for these data.
Also, every model in the game consist of some (not only one) components - 3d model, materials, textures, collision data, some other.
I mean that if you want to change some model (for house, for example) in the game - then you need to provide all neccessary data and files, not only one file.