diff --git a/synapse/media/_base.py b/synapse/media/_base.py
index ef8334ae2..aba6a0e71 100644
--- a/synapse/media/_base.py
+++ b/synapse/media/_base.py
@@ -184,7 +184,9 @@ def add_file_headers(
     # recommend caching as it's sensitive or private - or at least
     # select private. don't bother setting Expires as all our
     # clients are smart enough to be happy with Cache-Control
-    request.setHeader(b"Cache-Control", b"public,max-age=86400,s-maxage=86400")
+    request.setHeader(
+        b"Cache-Control", b"public,immutable,max-age=86400,s-maxage=86400"
+    )
     if file_size is not None:
         request.setHeader(b"Content-Length", b"%d" % (file_size,))