Python new test framework should be used (Refer this for more detail)

The edge case handling is not explained.

Your requirement has the wrong premise in the first place: the extension of a file is always the last part of a file name, never all the parts after the dot. .tar.gz simply means it's a g-zipped file (.gz) which has a tar file (.tar), so its extension is .gz, not .tar.gz. Similarly, innocent_file.txt.exe has an extension of .exe, not .txt.exe.

It probably needs test for paths without extension. Something like

test.assert_equals(separate_filename_from_extension('/foo/bar'), ('/foo/bar', ''))