You may have noticed that Confluence now opens attachments in its own built-in preview tool. Some users have expressed a desire to disable this feature, but unfortunately Confluence does not yet offer any way for a user to selectively disable this feature.
If you'd like to open an attachment and avoid this previews feature there is a workaround. When you click the link to open the attachment hold down the ctrl key (or cmd if you're using a mac). Using this shortcut will open the attachment in another browser tab. This should then execute your browser's default action for that type of file, or prompt you to decide whether to save or open that file in a program.