Currently, during the development mode, images and scripts are served directly by Jetty, which uses the file timestamp as the Last-Modified HTTP header and set the epoch time as the Expires header. This makes browsers come back later with 'If-Modified-Since", and Jetty checks that agsinst the local time stamp and returns "304 Not Modified".

When one goes between Jenkins branches, this can end up polluting cache — say you go back to a branch that contains the old version of a file, then your local cache from the newer version of Jenkins will win and you don't get the proper version of the file until you flush the cache.

The release versions don't have this problem as files are served under special /static/HASH/ URL. We should do the same with the development version.