[exim] dkim_verify_signers and file expansion

Hi everyone. I'm trying to set dkim_verify_signers by reading a list of
known signers in an external file.

As with lists, I tried the following:

dkim_verify_signers = $dkim_signers : /etc/exim/some_file

where the contents of "some_file" is a list of domain names:

gmail.com
yahoo.com
...

but it doesn't work as I would normally expect.
Instead, the following:

dkim_verify_signers = $dkim_signers : ${readfile{/etc/exim/some_file}{:}}

works. Can somebody explain why I need to expand the file contents
manually here?

I'm using exim 4.80 (-7, from debian unstable).

Thanks.