I am trying to understand Http/Https a little better and possibly what headers I'm sending clients.
Does the client have to re-fetch the same file under Https when it has already been fetched under Http, or do I need to send special headers?
Well, the reason the file is served over http/https is that it is simply a banner. When the user goes to a secure page, the banner is still in that particular template. Once the user does get the file via Https, the file is cached as expected.
Walter
https is considered a different "domain" from http and so if you send a file to a client over a http connection, then switch to https (even if you don't change anything else) the browser will still request the file again - as if the file wasn't in the cache. It's just like if you requested http://www.example1.com/file.txt and then http://www.example2.com/file.txt the might be the "same" file, but the browser doesn't know and so it's got to fetch it a second time.
Once it's requested it via https, though, it'll cache as normal.