Add option download_msg_attachments to choose protocol

I came across a situation where I wanted to save an attachment to the local bbs server, however because sbbs forces you to use the default protocol and not change it before doing the download, I can never accomplish that until I change the default protocol in User Settings to Email Download or don't select a default protocol at all.

Adding an option to download_msg_attachments that the script writer can use to force the protocol to be selected would be useful to bypass the auto download and force the user to select the protocol anyway.

This would be useful for, say Nightfox's DDMsgReader where users can save an attachment but may want to save to the bbs server.