This switches the behaviour from:

multipart/related
  multipart/alternative
    text/plain
    text/html
  attachment (disposition: inline, image1)
  attachment (disposition: attachment, file1)
  attachment (disposition: attachment, file2)

to:

multipart/mixed
  multipart/related
    multipart/alternative
      text/plain
      text/html
    attachment (disposition: inline, image1)
  attachment (disposition: attachment, file1)
  attachment (disposition: attachment, file2)

Fixes #2686.

Thanks to clemens and eGust for reviewing.