There is currently no way to search for the name of a file that is attached to one of your pages. There is a ListFiles module planned which would allow you to generate your own list of files in the site. Using a bit of customisation, you'd be able to order the results according to filename, filesize, etc. This page manager is a good example of how you can order a list of results.
As for backup — Wikidot regularly performs multiple backups of the entire wikifarm. I think there is 3-4 backup servers, with two of them being in separate geographical locations to the main server. That said, personal backups are available and can be found from the Site Manager (usually on the admin:manage page in your site). They backup the page source, and your uploaded files. Things like the parent-child relationship between pages, tags, and other metadata is not backed-up by this Site Manager function, so it's not perfect.
I can't see you ever having to reset back to a previous version of your site though - I doubt the content will be lost by Wikidot.
