diff --git a/lib/csslib.php b/lib/csslib.php
index a8217223cf2..109711f36d2 100644
--- a/lib/csslib.php
+++ b/lib/csslib.php
@@ -292,7 +292,7 @@ function css_send_cached_css($csspath, $etag) {
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', filemtime($csspath)) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Pragma: ');
-    header('Cache-Control: public, max-age='.$lifetime);
+    header('Cache-Control: public, max-age='.$lifetime.', immutable');
     header('Accept-Ranges: none');
     header('Content-Type: text/css; charset=utf-8');
     if (!min_enable_zlib_compression()) {
@@ -318,7 +318,7 @@ function css_send_cached_css_content($csscontent, $etag) {
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', time()) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Pragma: ');
-    header('Cache-Control: public, max-age='.$lifetime);
+    header('Cache-Control: public, max-age='.$lifetime.', immutable');
     header('Accept-Ranges: none');
     header('Content-Type: text/css; charset=utf-8');
     if (!min_enable_zlib_compression()) {
diff --git a/lib/editor/tinymce/plugins/loader.php b/lib/editor/tinymce/plugins/loader.php
index 141a6ee7b2c..537ac791d53 100644
--- a/lib/editor/tinymce/plugins/loader.php
+++ b/lib/editor/tinymce/plugins/loader.php
@@ -62,7 +62,7 @@ if ($allowcache) {
     // Set it to expire a year later. Note that this means we should never get
     // If-Modified-Since requests so there is no need to handle them specially.
     header('Expires: ' . date('r', time() + 365 * 24 * 3600));
-    header('Cache-Control: max-age=' . 365 * 24 * 3600);
+    header('Cache-Control: max-age=' . 365 * 24 * 3600 . ', immutable');
     // Pragma is set to no-cache by default so must be overridden.
     header('Pragma:');
 }
diff --git a/lib/filelib.php b/lib/filelib.php
index a3ca2cbd09c..a99d293b788 100644
--- a/lib/filelib.php
+++ b/lib/filelib.php
@@ -2155,6 +2155,7 @@ function file_safe_save_content($content, $destination) {
  *                        and should not be reopened.
  * @param array $options An array of options, currently accepts:
  *                       - (string) cacheability: public, or private.
+ *                       - (string|null) immutable
  * @return null script execution stopped unless $dontdie is true
  */
 function send_file($path, $filename, $lifetime = null , $filter=0, $pathisstring=false, $forcedownload=false, $mimetype='',
@@ -2195,6 +2196,14 @@ function send_file($path, $filename, $lifetime = null , $filter=0, $pathisstring
     }
 
     if ($lifetime > 0) {
+        $immutable = '';
+        if (!empty($options['immutable'])) {
+            $immutable = ', immutable';
+            // Overwrite lifetime accordingly:
+            // 90 days only - based on Moodle point release cadence being every 3 months.
+            $lifetimemin = 60*60*24*90;
+            $lifetime = max($lifetime, $lifetimemin);
+        }
         $cacheability = ' public,';
         if (!empty($options['cacheability']) && ($options['cacheability'] === 'public')) {
             // This file must be cache-able by both browsers and proxies.
@@ -2207,7 +2216,7 @@ function send_file($path, $filename, $lifetime = null , $filter=0, $pathisstring
             $cacheability = ' private,';
         }
         $nobyteserving = false;
-        header('Cache-Control:'.$cacheability.' max-age='.$lifetime.', no-transform');
+        header('Cache-Control:'.$cacheability.' max-age='.$lifetime.', no-transform'.$immutable);
         header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
         header('Pragma: ');
 
@@ -2294,6 +2303,8 @@ function send_file($path, $filename, $lifetime = null , $filter=0, $pathisstring
  *  (string|null) cacheability - force the cacheability setting of the HTTP response, "private" or "public",
  *      when $lifetime is greater than 0. Cacheability defaults to "private" when logged in as other than guest; otherwise,
  *      defaults to "public".
+ *  (string|null) immutable - set the immutable cache setting in the HTTP response, when served under HTTPS.
+ *      Note: it's up to the consumer to set it properly i.e. when serving a "versioned" URL.
  *
  * @category files
  * @param stored_file $stored_file local file object
diff --git a/lib/jslib.php b/lib/jslib.php
index d3114c104ae..c39ce4472c5 100644
--- a/lib/jslib.php
+++ b/lib/jslib.php
@@ -41,7 +41,7 @@ function js_send_cached($jspath, $etag, $filename = 'javascript.php') {
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', filemtime($jspath)) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Pragma: ');
-    header('Cache-Control: public, max-age='.$lifetime);
+    header('Cache-Control: public, max-age='.$lifetime.', immutable');
     header('Accept-Ranges: none');
     header('Content-Type: application/javascript; charset=utf-8');
 
diff --git a/lib/upgrade.txt b/lib/upgrade.txt
index ba7874fc69e..c7346c2bd4a 100644
--- a/lib/upgrade.txt
+++ b/lib/upgrade.txt
@@ -1,6 +1,10 @@
 This files describes API changes in core libraries and APIs,
 information provided here is intended especially for developers.
 
+=== 3.2.4 ===
+
+* The caching option 'immutable' has been added to send_stored_file() and send_file().
+
 === 3.2.3 ===
 
 * Behat compatibility changes are now being documented at
diff --git a/theme/font.php b/theme/font.php
index d6d5e22208a..c4a16eaddc5 100644
--- a/theme/font.php
+++ b/theme/font.php
@@ -180,7 +180,7 @@ function send_cached_font($fontpath, $etag, $font, $mimetype) {
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', filemtime($fontpath)) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Pragma: ');
-    header('Cache-Control: public, max-age='.$lifetime);
+    header('Cache-Control: public, max-age='.$lifetime.', immutable');
     header('Accept-Ranges: none');
     header('Content-Type: '.$mimetype);
     header('Content-Length: '.filesize($fontpath));
diff --git a/theme/image.php b/theme/image.php
index ed5080f1147..7af9ed6f0be 100644
--- a/theme/image.php
+++ b/theme/image.php
@@ -230,7 +230,7 @@ function send_cached_image($imagepath, $etag) {
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', filemtime($imagepath)) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Pragma: ');
-    header('Cache-Control: public, max-age='.$lifetime.', no-transform');
+    header('Cache-Control: public, max-age='.$lifetime.', no-transform, immutable');
     header('Accept-Ranges: none');
     header('Content-Type: '.$mimetype);
     header('Content-Length: '.filesize($imagepath));
diff --git a/theme/jquery.php b/theme/jquery.php
index f4527d2cf0d..da5e961dbf9 100644
--- a/theme/jquery.php
+++ b/theme/jquery.php
@@ -125,7 +125,7 @@ header('Content-Disposition: inline; filename="'.$filename.'"');
 header('Last-Modified: '. gmdate('D, d M Y H:i:s', filemtime($file)) .' GMT');
 header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
 header('Pragma: ');
-header('Cache-Control: public, max-age='.$lifetime);
+header('Cache-Control: public, max-age='.$lifetime.', immutable');
 header('Accept-Ranges: none');
 header('Content-Type: '.$mimetype);
 
diff --git a/theme/yui_combo.php b/theme/yui_combo.php
index 9e4e536e5ab..d237d441655 100644
--- a/theme/yui_combo.php
+++ b/theme/yui_combo.php
@@ -401,7 +401,7 @@ function combo_send_cached($content, $mimetype, $etag, $lastmodified) {
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', $lastmodified) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Pragma: ');
-    header('Cache-Control: public, max-age='.$lifetime);
+    header('Cache-Control: public, max-age='.$lifetime.', immutable');
     header('Accept-Ranges: none');
     header('Content-Type: '.$mimetype);
     header('Etag: "'.$etag.'"');
diff --git a/theme/yui_image.php b/theme/yui_image.php
index 5865aa37f7a..894ece8e932 100644
--- a/theme/yui_image.php
+++ b/theme/yui_image.php
@@ -123,7 +123,7 @@ function yui_image_cached($imagepath, $imagename, $mimetype, $etag) {
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', filemtime($imagepath)) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Pragma: ');
-    header('Cache-Control: public, max-age=315360000, no-transform');
+    header('Cache-Control: public, max-age=315360000, no-transform, immutable');
     header('Accept-Ranges: none');
     header('Content-Type: '.$mimetype);
     header('Content-Length: '.filesize($imagepath));