Send some expiry headers on some images served from thumb.php.

So far, they are only being sent on images that we know will never change, namely the profileiconbyid icons and thumbs for the blocktypes.

htdocs/thumb.php

@@ -44,9 +44,19 @@ switch ($type) {
 
         if ($id) {
             if ($path = get_dataroot_image_path('artefact/internal/profileicons', $id, $size)) {
-                $type = get_mime_type($path);
-                if ($type) {
-                    header('Content-type: ' . $type);
+                $mimetype = get_mime_type($path);
+                if ($mimetype) {
+                    header('Content-type: ' . $mimetype);
+
+                    // We can't cache 'profileicon' because it might change, 
+                    // but 'profileiconbyid' never changes...
+                    if ($type == 'profileiconbyid') {
+                        // 1 week
+                        $maxage = 604800;
+                        header('Expires: '. gmdate('D, d M Y H:i:s', time() + $maxage) .' GMT');
+                        header('Cache-Control: max-age=' . $maxage);
+                        header('Pragma: public');
+                    }
                     readfile($path);
                     exit;
                 }
@@ -98,6 +108,10 @@ switch ($type) {
             $basepath = 'artefact/' . $ap . '/' . $basepath;
         }
         header('Content-type: image/png');
+        $maxage = 604800;
+        header('Expires: '. gmdate('D, d M Y H:i:s', time() + $maxage) .' GMT');
+        header('Cache-Control: max-age=' . $maxage);
+        header('Pragma: public');
         $path = get_config('docroot') . $basepath . '/thumb.png';
         if (is_readable($path)) {
             readfile($path);