The file manager used in HTML Editor is a 3rd party File manager called KFM:
the problem is that they have not released an update to support IE 10 that we can use in HTML Editor.
you can force the file manager to use the IE9 or IE 8 engine by editing webasssist/kfm/index.php file. In the head section add this line of code:
<meta http-equiv="x-ua-compatible" content="IE=9" >
or you can use:
<meta http-equiv="x-ua-compatible" content="IE=8" >