How can I prevent Bing from indexing attachment URLs?
This type of behaviour is expected when the option "Require authentication to download" is not enabled. To enable this feature click the Admin icon ( ) in the sidebar, then select Settings > Tickets. On this page you will have the option to select Require authentication to download.
If the option is enabled, then the next time these links are crawled by Bing, or other other Search bots, the URLs will not be indexed anymore.
If the links have already been indexed by Bing and you want to have them removed, you will need to submit a content removal request to Bing.