File Sharing is off in iTunes

If I connect my iPhone to my laptop by USB and launch iTunes, and then I go to File Sharing, I get an empty panel and the message: File Sharing is Off.

I cannot understand where and how can I enable it. Is that an iTunes or an iPhone setting?

PS. Please, avoid the automated message that tells me how to transfer files by iTunes. This is a precise question and I need a precise answer. :-)