This adds the ability to scan all installed media files (or a subset of them) to find the last modified file's timestamp. This is used as a media serial suffix, appended to filenames as a query argument (for example, foo.png?20081228). This can be used to add persistent browser-side caching for media files. Each of these files can be set with an Expires of 2 years, or some other large amount of time, without the worry of files going stale after being updated.

This is used by an accompanying change in Review Board.

Used this with my change in Review Board on Apache, and saw that the browser handled persistent caching of media files.