Plack::Middleware::Precompressed is an alternative (or rather,
complement) to middlewares like Deflater, which will compress response
bodies on the fly.  For dynamic resources, that behaviour is
necessary, but for static resources it is a waste: identical entities
will be compressed over and over.  Instead, Precompressed allows you
to compress static resources once, e.g. as part of your build process,
and then serve the compressed resource in place of the uncompressed
one for compression-enabled clients.

To do so, it appends a .gz suffix to the request URI and tries to
serve that.  If that fails, it will try again with the unmodified URI.