There are cases where a user can copy image markdown from a public
post (such as via the discourse-templates plugin) into a PM which
is then sent via an email. Since a PM is a secure context (via the
.with_secure_uploads? check on Post), the image will get a secure
URL in the PM post even though the backing upload is not secure.
This fixes the bug in that case where the image would be stripped
from the email (since it had a /secure-uploads/ URL) but not re-attached
further down the line using the secure_uploads_allow_embed_images_in_emails
setting because the upload itself was not secure.
The flow in Email::Sender for doing this is still not ideal, but
there are chicken and egg problems around when to strip the images,
how to fit in with other attachments and email size limits, and
when to apply the images inline via Email::Styles. It's convoluted,
but at least this fixes the Template use case for now.
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