Guess the type of a file based on its URL.
Return value is a tuple (type, encoding) where type is None if the
type can't be guessed (no or unknown suffix) or a string of the
form type/subtype, usable for a MIME Content-type header; and
encoding is None for no encoding or the name of the program used
to encode (e.g. compress or gzip). The mappings are table
driven. Encoding suffixes are case sensitive; type suffixes are
first tried case sensitive, then case insensitive.
The suffixes .tgz, .taz and .tz (case sensitive!) are all mapped
to ".tar.gz". (This is table-driven too, using the dictionary
suffix_map).
Optional 'strict' argument when false adds a bunch of commonly found, but
non-standard types.