diff --git a/gerrit-gwtui/src/main/java/net/codemirror/mode/mode_map b/gerrit-gwtui/src/main/java/net/codemirror/mode/mode_map
index e448fa9a62..c3128f6911 100644
--- a/gerrit-gwtui/src/main/java/net/codemirror/mode/mode_map
+++ b/gerrit-gwtui/src/main/java/net/codemirror/mode/mode_map
@@ -52,4 +52,5 @@ text/xml
 application/xml
 
 application/x-javascript = application/javascript
+text/x-h = text/x-c++hdr
 text/x-java-source = text/x-java