Followup 96a0781bc1
When sending emails where secure uploads is enabled
and secure_uploads_allow_embed_images_in_emails is
true, we attach the images to the email, and we
do some munging with the final email so the structure
of the MIME parts looks like this:
```
multipart/mixed
multipart/alternative
text/plain
text/html
image/jpeg
image/jpeg
image/png
```
However, we were not specifying the `boundary` of the
`multipart/mixed` main content-type of the email, so
sometimes the email would come through appearing to
have an empty body with the entire thing attached as
one attachment, and some mail parsers considered the
entire email as the "epilogue" and/or "preamble".
This commit fixes the issue by specifying the boundary
in the content-type header per https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html
a128ce5c4c