gr.group theory - nilpotent matrices over polynomial rings

I am looking for an analogue of the Jordan normal form for nilpotent matrices over the
polynomial ring ${mathbb Z}[x_1, dots, x_n]$. More precisely, is there a description for the orbits of action by conjugation of $GL_m({mathbb Z}[x_1, dots, x_n])$ on $M_{m times m}({mathbb Z}[x_1, dots, x_n])$?