ChatMessageValidator: Allow one message in an array, too.

lib/pleroma/web/activity_pub/object_validators/chat_message_validator.ex

@@ -47,9 +47,18 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.ChatMessageValidator do
   def fix(data) do
     data
     |> fix_emoji()
+    |> fix_attachment()
     |> Map.put_new("actor", data["attributedTo"])
   end
 
+  # Throws everything but the first one away
+  def fix_attachment(%{"attachment" => [attachment | _]} = data) do
+    data
+    |> Map.put("attachment", attachment)
+  end
+
+  def fix_attachment(data), do: data
+
   def changeset(struct, data) do
     data = fix(data)