How can I prevent Bing from indexing attachments URLs?
This type of behaviour is expected when the option "Require authentication to download" is not enabled.
You would need to navigate to navigate to SETTINGS - Tickets and enable the option 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 should submit a content removal request to Bing.