how to get a filename's extension

[SuperGh0d at gmail.com]

Hello all,

I am new to python and I am having some difficulty obtaining the
extension of a file. I was looking at the re.match module but I am
completely overwhelmed by regular expressions. Is there an easier way
to get the extension for a file name like:

this.is.my.file.avi

I would just like to pull out the 'avi'

your help is greatly appreciated! :)