We set custom mimetypes in the e-mail previews for HTML and plain text,
but these weren't specifying a character set, causing the browser to
fall back on a default and render UTF-8 content incorrectly. These
mimetypes have been updated to explicitly specify the UTF-8 character
set, fixing rendering.

Testing Done:
Viewed UTF-8 content in the e-mail previews. Saw that they now render
correctly.

Reviewed at https://reviews.reviewboard.org/r/8902/