FIX: Chat uploads over-secured in some situations (#29586)

In the case where:

* Secure uploads were enabled
* Allow unsecure chat uploads was enabled
* For a site with login required enabled

When a chat upload was created, it was being marked as secure. Since
there is no provision for secure uploads in chat, this would lead to
broken uploads/images shown in the channel.

We can use the "public types" functionality of secure uploads to make
sure we never mark chat uploads as secure, and we can revisit this
whenever we get around to allowing secure uploads in chat.

plugins/chat/plugin.rb

@@ -548,6 +548,10 @@ after_initialize do
   )
 
   register_bookmarkable(Chat::MessageBookmarkable)
+
+  # When we eventually allow secure_uploads in chat, this will need to be
+  # removed. Depending on the channel, uploads may end up being secure.
+  UploadSecurity.register_custom_public_type("chat-composer")
 end
 
 if Rails.env == "test"