gptel: Improve binary file detection with multibyte check

* gptel.el (gptel--file-binary-p): Detect multibyte coding-systems
used for binary files in `gptel--file-binary-p'.  Should help with
problems like #914 and possibly #912.

Author: karthink

gptel.el

@@ -976,7 +976,8 @@ Later plists in the sequence take precedence over earlier ones."
   (condition-case nil
       (with-temp-buffer
         (insert-file-contents path nil 1 512 'replace)
-        (eq buffer-file-coding-system 'no-conversion))
+        (memq buffer-file-coding-system
+              '(no-conversion no-conversion-multibyte)))
     (file-missing (message "File \"%s\" is not readable." path)
                   nil)))