The iso-8859 character sets are all 8bit, but are still quoted-printable encoded. Why? See below procmime.c:1200

EncodingType procmime_get_encoding_for_charset(const gchar *charset)
{
        if (!charset)
                return ENC_8BIT;
        else if (!g_ascii_strncasecmp(charset, "ISO-2022-", 9) ||
                 !g_ascii_strcasecmp(charset, "US-ASCII"))
                return ENC_7BIT;
        else if (!g_ascii_strcasecmp(charset, "ISO-8859-5") ||
                 !g_ascii_strncasecmp(charset, "KOI8-", 5) ||
                 !g_ascii_strcasecmp(charset, "Windows-1251"))
                return ENC_8BIT;
        else if (!g_ascii_strncasecmp(charset, "ISO-8859-", 9))
                return ENC_QUOTED_PRINTABLE;
        else if (!g_ascii_strncasecmp(charset, "UTF-8", 5))
                return ENC_QUOTED_PRINTABLE;
        else
                return ENC_8BIT;
}