Images corrupted in PDF in response

[RichardTaylor]

See: https://www.whatdotheyknow.com/request/newly_overflown_postcodes_during#incoming-1662245

Images render correctly showing maps if the raw email is opened in another mail application.

Possible but unconfirmed example of the issue at #239

Looks different to the issue at #5905

I recall a similar issue with images on a similar subject before.