diff --git a/config/config.exs b/config/config.exs
index 3704be0ed..78bf04ddc 100644
--- a/config/config.exs
+++ b/config/config.exs
@@ -65,6 +65,10 @@ config :mime, :types, %{
   "application/xrd+xml" => ["xrd-xml"]
 }
 
+config :mime, :extensions, %{
+  "activity-json" => "application/activity+json"
+}
+
 # Upload configuration
 config :mobilizon, Mobilizon.Web.Upload,
   uploader: Mobilizon.Web.Upload.Uploader.Local,