add correct content type for catch-all

lib/pleroma/web/router.ex

@@ -107,5 +107,11 @@ def user_fetcher(username) do
 
 defmodule Fallback.RedirectController do
   use Pleroma.Web, :controller
-  def redirector(conn, _params), do: (if Mix.env != :test, do: send_file(conn, 200, "priv/static/index.html"))
+  def redirector(conn, _params) do
+    if Mix.env != :test do
+      conn
+      |> put_resp_content_type("text/html")
+      |> send_file(200, "priv/static/index.html")
+    end
+  end
 end