> What files are supported if I want to upload some 3d models?

It's pretty open.  I don't know that there is a universal standard for 3D object files, so whatever format you are comfortable with should be fine.  

> Can I also upload 3D models from leaked cancelled PC games?

No.

This site is for publicly licensed works legally released under terms of any of the supported licenses.

Fan art, ripped sprites, models, etc. from commercial or otherwise closed license projects is not alowed.

> Is it allowed to upload custom made 3d models that are made by me on this website?

Yes, so long as they are new and original creations by you.  They should not depcit or be derived from any copyrighted or trademarked characters or materials.

Hope that helps!