Intercept and use local files in http requests

Question

I'm trying to find a tool that will allow non-programmers to test files on a live server.

For example, they could modify an image on their computer, reload a webpage, then see the results of their work immediately.

I've tried finding a tool for this, because it seems obvious enough that someone must've thought of it, but a lot of software I see doesn't quite fit. A tool called Fiddler does this (they call it autoresponding) but it's Windows-only. I could change the hosts file to redirect to a local instance of nginx or something, but that seems difficult to maintain when all I really want is a simple tool that will something like this...

http://someserver.com/css/(.*) -> /home/user/localcss/$1

Does anybody have any recommendations?

Edit: Redirect clarification

Answer 1

Fiddler has this feature; just click the AutoResponder tab and map URLs to local files. Thousands of people do this every day.

See also video #5 here: http://www.fiddlerbook.com/fiddler/help/video/default.asp

Answer 2

I found Charles Proxy very useful for this http://www.charlesproxy.com/documentation/tools/map-local/