From what I'm aware models should be in common formats such as .obj or .dae, with textures as .mdl or just regular graphic formats. It's similar to submitting zipped sprites, they should be in usable formats like .png or .gif just like the sheets, so that people can use them easily.
I also think the models need to be rigged? Not completely sure, this is the one site I don't submit to
I also think the models need to be rigged? Not completely sure, this is the one site I don't submit to