In case of a single-part mail, the charset is taken from the body. In a multi-part mail, this could get tricky if the parts have different charsets.
Bug 1779070 Comment 8 Edit History
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So the subject is displayed to me as "koi8-r". In case of a single-part mail, the charset is taken from the body. In a multi-part mail, this could get tricky if the parts have different charsets.