=== modified file 'sites/all/modules/filefield/field_file.inc'
--- sites/all/modules/filefield/field_file.inc	2010-07-01 14:24:51 +0000
+++ sites/all/modules/filefield/field_file.inc	2010-07-28 19:14:45 +0000
@@ -156,7 +156,13 @@
   // Call the validation functions.
   $errors = array();
   foreach ($validators as $function => $args) {
-    array_unshift($args, $file);
+    // We have to pass $file as a reference (because call_user_func_array()
+    // ignores function definitions; it's the only function where we must use
+    // call-time pass-by-reference).
+    // Since we cannot safely use call-time references anywhere else
+    // (including array_unshift), we do it like this:
+    array_unshift($args, NULL);
+    $args[0] = &$file;
     $errors = array_merge($errors, call_user_func_array($function, $args));
   }