Some shy away from setting an expire time on images because they often update
images in place, particularly recently-created images. This addition to
mod_expire addresses the latter case, allowing one to set different expire times
depending on whether the file being served is/isn't older than some set threshold.

For example, to set the expire time far into the future for JPG files that are
more than a week old:

   ExpiresByType image/jpeg "aged 1 week then 10 years"

This leaves JPG images less than a week old without an expire tag, and ones a
week old or older with one 10 years in the distance (from the access time).

It also supports an "else" clause:

  ExpireByType image/jpeg "aged 1 week then 10 years else 1 hour"

The patch does not include the documentation -- I wanted to gauge the reaction,
first.

This small patch can go a long way toward allowing web sites to get the benefits
of image caching, while still allowing a window for in-place updates. This can
also be useful for CSS and JS, in conjunction with filename versioning.