[Exim] message_body_size bug?

I have encountered what looks like a bug with the $message_body_size filter
variable, using version 3.33. To demonstrate it I used the filter:

# Exim filter

if $message_body_size is below 5k
then
fail text "Too small: $message_body_size ($message_size)"
seen finish
endif

and the used

exim -bF < testmessage

and got:

Fail text "Too small 0 (78906)"

There really is a $message_body in that message because I can do other filter
operations on it.

Phil.
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Phil Chambers (postmaster@???)
University of Exeter