Attachments do not send or honour text/plain;charset=UTF-8 encoding

'text/plain' attachments are served with charset ISO-8859-1 even if they are UTF-8 (the common case). Here is an example:

  https://launchpadlibrarian.net/118025172/airplane-mode-translations.txt

the output looks corrupted, unless the viewer manually overrides their browser Encoding. Additionally even providing the '; charset=UTF-8' suffix on the attachment edit page:

  https://bugs.launchpad.net/ayatana-design/+bug/1034488/+attachment/3362298

does not serve it as UTF-8. Ideally:

  Launchpad Librarian should default to UTF-8 for textual attachments
  Launchpad Librarian should respect and pass through the '; charset=' suffix as provided.

Header example:

  wget -q -S https://launchpadlibrarian.net/118025172/airplane-mode-translations.txt -O /dev/null 2>&1 | grep Content-Type:
  Content-Type: text/plain