Well, as I said, if it's a separate mail client, it's unlikely it would work for any conversation attachments - e.g. attachments uploaded to your server. Unless you have physically logged into your forum via your mail client, you'll be treated as a guest. Guests won't be able to view attachments which are attached to a conversation you're part of. If you have the view attachments permission enabled for guests and an attachment is attached to a post in a guest visible forum, and you receive a watched thread reply email, they'll likely show properly.