Index: sites/all/modules/private_upload/private_upload.module
===================================================================
--- sites/all/modules/private_upload/private_upload.module	(revision 2728)
+++ sites/all/modules/private_upload/private_upload.module	(working copy)
@@ -745,18 +745,23 @@
  * Create a URL for the file that changes if the file is public or private.
  * TODO - Push to get a file_create_url hook into Drupal7.
  *
  * @param file object $file
  * @return str: the correct URL
  */
 function _private_upload_create_url($file) {
+  global $conf;
   if (_private_upload_is_file_private($file->filepath)) {
     $download_method = variable_get('file_downloads', FILE_DOWNLOADS_PUBLIC); // this should be PUBLIC, but don't break misconfigured systems
-    variable_set('file_downloads', FILE_DOWNLOADS_PRIVATE);
+    $conf['file_downloads'] = FILE_DOWNLOADS_PRIVATE;
   }
   // Generate valid URL for both existing attachments and preview of new attachments (these have 'upload' in fid)     
   $href = file_create_url((strpos($file->fid, 'upload') === FALSE ? $file->filepath : file_create_filename($file->filename, file_create_path())));
   if (_private_upload_is_file_private($file->filepath)) {
-    variable_set('file_downloads', $download_method);
+    $conf['file_downloads'] = $download_method;
   }
   return $href;
 }