Symptom : using the MailStep from workflow-plugin, the generated email always has the "charset=UTF-8" in the Content-Type part header, regardless of the charset parameter, but the non-ascii characters are not properly escaped.
e.g. the word "Synthèse" in the body is escaped as "Synth=E8se" instead of "Synth=C3=A8se".

Looking at the source code for MimeMessageBuilder, the charset parameter is indeed not used for the content, but only for addresses.