fix(3p/lisp/mime4cl): support FILE-PORTION in PRINT-MIME-PART

Change-Id: I942e8915d5076628179dfa77bf80b7510b862b51
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12855
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

third_party/lisp/mime4cl/mime.lisp

@@ -878,6 +878,8 @@ returns a MIME-MESSAGE object."
        (loop
           for byte across body
           do (write-char (code-char byte) out)))
+      (file-portion
+       (redirect-stream (open-decoded-file-portion body) out))
       (pathname
        (with-open-file (in body)
          (redirect-stream in out))))))