What type of biome is this?
Well, first of all 812mm is a pretty precise value to be preceded by the word "about". "Mildly hot", on the other hand, is too vague a term for your question to be answerable. If you update your question with better parameters, you may get more helpful answers.

In any case, according to this page, a tropical savanna has mean monthly temperatures at or above 64°F (~18°C), which may or may not qualify as "mildly hot" by your standards, and annual precipitation averages between 30 and 50 inches which means between 762mm and 1270mm.  

So, I what you are describing could be a tropical savanna, but as I said before, I can't tell unless you update your question with more information. Have a look at this site, it may be of help.