Encoding::UndefinedConversionError: "\xC3" from ASCII-8BIT to UTF-8

```
[2017-06-12T12:35:13.472] FATAL -- : ActionView::Template::Error (incompatible character encodings: UTF-8 and ASCII-8BIT):
[2017-06-12T12:35:13.472] FATAL -- : 15: or the e-mail didn't include a plain text part.
16: - else
17: %pre.codeBlock= @message.plain_body
[2017-06-12T12:35:13.472] FATAL -- :
[2017-06-12T12:35:13.472] FATAL -- : app/views/messages/plain.html.haml:18:in `_app_views_messages_plain_html_haml__809546143829012049_70075317594600'
```

The e-mail contains a body / text part.

I can't find the character that causes the issue in the raw EML download.

Possible solution to force encoding ?

Start at app/views/messages/plain.html.haml and trace how @message.plain_body is produced for the reported EML message. Reproduce the rendering failure using the raw EML download and identify the expected handling for the incompatible encoding; done means this message renders without the reported conversion error.

Written by the indexing model from the issue text.