Override proxy's incorrect Content-Type via mime settings

[Jonathan Matthews]

Hi all -

I have an upstream proxy that sends the wrong Content-Type header for
a load of images. Sometime it doesn't send any header at all.

I'm trying to get Nginx not only to remove the upstream's Content-Type
header ("proxy_hide_header"), but also to insert the correct header.
Unfortunately, I can't hard-code the value as there are a variety of
image formats being used. "proxy_hide_header" doesn't seem to do
anything other than block the incorrect value, and
"proxy_ignore_headers" won't accept "Content-Type".

How can I both remote upstream's incorrect header, and also add the
header than Nginx *would* have used if it were serving the content
from local disk?
In other words, how do I add the header with a value derived from the
MIME settings currently configured, via the requested file's
extension?

Version 0.7.67, but can upgrade if required.

Many thanks,
Jonathan
-- 
Jonathan Matthews
London, UK
http://www.jpluscplusm.com/contact.html