utils: fix another possible leak with private S3 backends using mediaproxy

same rationale as the other mediaproxy changes

lib/pleroma/web/common_api/utils.ex

@@ -2,6 +2,7 @@ defmodule Pleroma.Web.CommonAPI.Utils do
   alias Pleroma.{Repo, Object, Formatter, Activity}
   alias Pleroma.Web.ActivityPub.Utils
   alias Pleroma.Web.Endpoint
+  alias Pleroma.Web.MediaProxy
   alias Pleroma.User
   alias Calendar.Strftime
   alias Comeonin.Pbkdf2
@@ -90,6 +91,7 @@ def add_attachments(text, attachments) do
       Enum.map(attachments, fn
         %{"url" => [%{"href" => href} | _]} ->
           name = URI.decode(Path.basename(href))
+          href = MediaProxy.url(href)
           "<a href=\"#{href}\" class='attachment'>#{shortname(name)}</a>"
 
         _ ->