As an engineer, anything that had "engineering" in the title, ie BioE, Mechanical Engineering, etc., I just put under engineering on my AMCAS.
If Biostats is in the department of stats, I would list it under math (there is no stats category on the AMCAS) and if it's in the department of Biology, I'd just put it under bio.
If the AMCAS people disagree with you, they'll probably just change the designation.