You've tagged this question with "Cloud", where the question is pretty much irrelevant, as it's not your server and not up to you how the attachments work.
If you are referring to your own servers, then I'm not sure the question is that useful. What do you mean by "management the disk usage"? What specifically do you want to know about the way Confluence uses space for attachments and for what reason? I can tell you that when you attach a file, Confluence sticks it on the disk in a tree structure that suits it, and places a line in the database so it knows how to use the file, but even that isn't going to help you manage disks. (Unless you've got attachments in the database, in which case, it's large objects in a table)
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