I'd offer to try disabling Last Modified header from your nginx and give
reasonable high (30 days?) value in Expires header.

Meanwhile you may report this error to whoever is assuming that absence
of Content-Type is reason for not processing your css (facebook or
facebook app developers, I'm not sure who you're talking about but IMHO
they all will at least comment on RFC position about this).

If you'll look at different servers sending 304 than you'll see that
google for example does not send Content Type. As well as apache.org
It's perfectly valid and accepted practice.

Alex.


On Wed, 2010-09-01 at 18:02 -0700, kevin wrote:
> it is for a facebook app, and i do not have control over this. without
> this header the css file is not processed as css at all.
>
>


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