stream-external-format
stream => format
stream—a file stream.
format—an external file format.
Returns an external file format designator for the stream.
(with-open-file (stream "test" :direction :output) (stream-external-format stream)) => :DEFAULT OR=> :ISO8859/1-1987 OR=> (:ASCII :SAIL) OR=> ACME::PROPRIETARY-FILE-FORMAT-17 OR=> #<FILE-FORMAT :ISO646-1983 2343673>
the :external-format argument to the function open and the with-open-file macro.
The format returned is not necessarily meaningful to other implementations.