[exim] running an external program on messages data

I am very much afraid that this is a FAQ, and in fact I may have asked
it myself once. Please forgive me, my memory definitely isn't what it
used to be.

What is the recommended/canonical way to run an arbitrary external
process on a mail that is being processed and capture the output of the
process so it can be used for expansion? I know about the following,
but none of it seems to fit:

= the run expansion item; sadly, doesn't get mail on stdin.

- the message_headers_raw, message_body and message_body_end variables.
It would be tricky and error prone just to paste all the pieces back
together to get the whole mail, even if they preserved the data
exactly (they don't).

- the transport_filter facility; modifies mail in place instead of
returning output.

Is there a way to get the file name(s) for the data in the queue? That
would mostly solve my problem.

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