url-uncompressor-alist

url-uncompressor-alist is a variable defined in `url-vars.el'.
Its value is

((".z" . "x-gzip")
 (".gz" . "x-gzip")
 (".uue" . "x-uuencoded")
 (".hqx" . "x-hqx")
 (".Z" . "x-compress")
 (".bz2" . "x-bzip2")
 (".xz" . "x-xz"))

Documentation:
An alist of file extensions and appropriate content-transfer-encodings.

You can customize this variable.