18.5.1 Using complete external formats

If open or with-open-file gets a complete :external-format argument then, it is used as is. For example, this form opens an ASCII linefeed-terminated stream:

(with-open-file (ss "C:/temp/ascii-lf"

                    :direction :output 

                    :external-format 

                    '(:ascii :eol-style :lf)) 

  (stream-external-format ss))

=>

(:ASCII :EOL-STYLE :LF)

If you know the encoding of a file you are opening, then you should pass the appropriate :external-format argument.