Convert text parts to utf-8
Those operators who convert text attachments to utf-8, seem to only understand utf-8 as a charset. If they see utf-8, they do nothing; if they see anything else, they interpret that as us-ascii and convert it to utf-8 (which typically messes things up).
Therefore, we need to convert text parts into utf-8 and specify utf-8 charset in the attachment part header. That way it should always work.
This automatic utf-8 conversion can be turned off with a command line option, or via config file. By default, conversion is on.