una/template/file_handler_code.html

<textarea class="bx_file_handler_code">__code__</textarea>
<script>
    var e = CodeMirror.fromTextArea($("textarea[class=bx_file_handler_code]").get(0), {
        lineNumbers: true,
        mode: "__mime_type__",
        htmlMode: true,
        matchBrackets: true,
        readOnly: true
    });
</script>