diff --git a/program/include/rcmail_output_html.php b/program/include/rcmail_output_html.php
index 0552b89c2..1b84e1b4e 100644
--- a/program/include/rcmail_output_html.php
+++ b/program/include/rcmail_output_html.php
@@ -1013,13 +1013,13 @@ EOF;
         // use minified file if exists (not in development mode)
         if (!$this->devel_mode && !preg_match('/\.min\.' . $ext . '$/', $file)) {
             $minified_file = substr($file, 0, strlen($ext) * -1) . 'min.' . $ext;
-            if ($fs = @filemtime($this->assets_dir . $minified_file)) {
-                return $minified_file . '?s=' . $fs;
+            if ($fs = @md5_file($this->assets_dir . $minified_file)) {
+                return $minified_file . '?s=' . substr($fs, 0, 6);
             }
         }
 
-        if ($fs = @filemtime($this->assets_dir . $file)) {
-            $file .= '?s=' . $fs;
+        if ($fs = @md5_file($this->assets_dir . $file)) {
+            $file .= '?s=' . substr($fs, 0, 6);
         }
 
         return $file;