Error saving edited node on server running with IIS and private files

I have a site running on IIS 6.0/Windows Server 2003 configured with private files (Drupal file handling). Private files directory contains \ (Windows directory separator).
I use SWFUpload with FileField. SWFUpload works OK when creating nodes.
If I edit a node containing a file without changing the file, I get an error when saving content : Fatal error: Cannot unset string offsets in path_to_file\swfupload_widget.inc on line 63

The error come from json_decode failure to decode the string passed by the form because it contains backslash (\) in the filepath value (e.g. "filepath":"D:\inetpub\wwwroot\mysite\files/sample.txt").

I have written a workaround. I don't know if this is the right thing to do but it works.

function swfupload_widget_value($element, $edit = FALSE) {
  if (is_string($edit)) {
    // original code - return null if string contains unescaped \
    //$edit = json_decode($edit, TRUE);
    // modified code
    $edit = json_decode(str_replace("\\", "\\u005c", $edit), TRUE);
  }