The File Cabinet page and Attachment section in Google Sites does not offer any way to sort the list of files. These instructions tell you how to use a List Page to link to the files and then you can choose to sort.
You will find these instructions easier to follow if you know how to use multiple browser tabs or windows.
There has been reports by some users that there can be a delay between adding the file, or attachment, to your site and it appearing in the page search results: if that's the case then you can right-click on Download and copy the URL of the file, or attachment, (Copy link location in Firefox, Copy shortcut in Internet Explorer, Copy link address in Google Chrome, Copy link in Safari) and add it to the URL column instead of using the search.
If you want to see what you can achieve with this method see our simple example: Example List Page as Front-end for File Cabinet.