'If the url is : "http://www.foobar.com/asd.txt" this method returns:
- http://www.foobar.com/asd.zip
- http://www.foobar.com/asd.tgz
- etc...
:param url: A URL to transform.
:return: A list of URL\'s that mutate the original url passed as parameter.
>>> from w3af.core.data.parsers.doc.url import URL
>>> u = url_fuzzer()
>>> list(u._mutate_file_type(URL(\'http://www.w3af.com/\')))
>>> url = URL(\'http://www.w3af.com/foo.html\')
>>> mutants = list(u._mutate_file_type( url))
>>> URL(\'http://www.w3af.com/foo.tar.gz\') in mutants
True
>>> URL(\'http://www.w3af.com/foo.disco\') in mutants
True
>>> len(mutants) > 20
True'

def _mutate_file_type(self, url):

    extension = url.get_extension()
    if extension:
        for filetype in chain(self._backup_exts, self._file_types):
            url_copy = url.copy()
            url_copy.set_extension(filetype)
            (yield url_copy)
