Guess the extension for a file based on its MIME type.
Return value is a string giving a filename extension, including the
leading dot ('.'). The extension is not guaranteed to have been
associated with any particular data stream, but would be mapped to the
MIME type 'type' by guess_type(). If no extension can be guessed for
'type', None is returned.
Optional 'strict' argument when false adds a bunch of commonly found,
but non-standard types.