[exim] syntax and use of ${run}

Where can I find some documentation on the syntax of ${run ...}?

I can't find it in the exim spec.

Then again, maybe there's a better way to do this.

I am trying to include the contents of a file in a header so I've been
trying:

headers_add = "My-Header: ${run {/bin/cat /my/file}{$value}{error}}"

All I get is is either 'error' or a blank, depending on how I configure it.

What's the best way to include a message from a file in the headers?