How to get file extension correctly?

I know that this question is asked many times on this website. But I found that they missed an important point: only file extension with one period was taken into consider like *.png *.mp3, but how do I deal with these filename with two period like .tar.gz.

The basic code is:

filename = '/home/lancaster/Downloads/a.ppt'
extention = filename.split('/')[-1]

But obviously, this code do not work with the file like a.tar.gz. How to deal with it? Thanks.