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.