In general, when common users open an attachment in a page, it will be either opened or saved. For users that are more advanced and IT-savvy, they may open it in a new tab by holding (Ctrl/Shift/Command) depending on browser. There are also KB and answers giving guide on how to open attachments in new tab automatically.

• How do I make my attachments open in a new window or a tab?
• How to make attachments open in new tabs?

However, based on customer experience, setting the Javascript or HTML tag will result opening new tabs for all pages and link.

It would be great if Confluence has a configuration to define whether attachments can be open in a new tab or not. (viewing pages and other links will not be affected)