Cached image's files by Mime-Type

[Samuel Martin]

Hello,

I'm trying to cache image by Mime-Type because behind varnish i've a
Zope/Plone architecture with some pictures without extension.

but this declaration in my vcl file doesn't working ::
        if (req.http.header ~ "image*" ) {
               lookup;
        }

contrary to ::
        if (req.url ~ "\.(css|js|gif|jpg|png|jpeg)$") {
                lookup;
        }

How i must interprete this ? Is my syntax bad or other ?

Thanks