Here is how to extract extension from filename in Python.

import os
file_name = "example.txt"
base, ext = os.path.splitext(file_name)
print(ext) 

.txt

You can extract the extension from a file name in Python using the os.path module, specifically the os.path.splitext() function. This function splits a file path into a tuple containing the file name and extension. 

Alternatively, you can also use the str.rsplit() method to extract the extension. Here's an example:

file_name = "example.txt"
ext = file_name.rsplit(".",1)[-1]
print(ext) # Prints 'txt'

Both of these methods will return the extension of the file, including the '.' if there is one.