Send accentuated characters inside an email body, how to fix encoding issues - ruby

Question

In a script which sends emails through Net::SMTP, I've to figure out how to properly encode the email body in order to support accentuated characters. I've separated the email into 3 parts: headers, body and attachment, as from this tutorial https://www.tutorialspoint.com/ruby/ruby_sending_email.htm
Fixing this issue for the Subject header field wasn't a big deal:
require 'base64'
MARKER = 'FOOBAR'
def self.headers
<<~EOF
From: someemail#domain.tld
To: anotheremail#domaim.tld
# Base64 encoded UTF-8
Subject: =?UTF-8?B?#{Base64.strict_encode64('Accentuated characters supportés')}?=
Date: #{Time.now.to_s}
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=#{MARKER}
--#{MARKER}
EOF
end
I was tempted to reproduce the same logic for the email body, without any success. I've try several headers, such as Language, Content-Language, Content-Tranfer-Encoding, again, without any success. Using Ruby's .encode!('utf-8') was also ineffective.
The only working solution I can think of would be to send HTML encoded characters: using &eacute instead of é inside a HTML block. Though, I'd like to avoid this solution as I've to improve my comprehension of encoding issues.
Does anyone has a suggestion about this issue ?
Here's my code so far, if it can help anyone:
module Reports
module SMTP
MARKER = 'FOOBAR'
def self.send_report(file_path)
file_content = File.binread(file_path)
encoded_content = [file_content].pack('m') # Base64
mail_content = headers + body + attachment(file_path, encoded_content)
begin
Net::SMTP.start('my.smtp.srv', 25, 'HELO_fqdn', 'username', 'p455w0rD', :plain) do |smtp|
smtp.send_message(mail_content, 'from#domain.tld', ['to1#domain.tld', 'to2#domain.tld'])
end
rescue => e
puts e.inspect, e.backtrace
end
end
def self.headers
# see above
end
def self.body
<<~EOF
Content-Type: text/html
Content-Transfer-Encoding:utf8
Here's a franglish text with some characters accentués
--#{MARKER}
EOF
end
def self.attachment(file_path, encoded_content)
<<~EOF
Content-Type: multipart/mixed; name = #{file_path}
Content-Transfer-Encoding:base64
Content-Disposition: attachment; filename = #{file_path}
#{encoded_content}
--#{MARKER}--
EOF
end
end
end
Note: these emails are correctly decoded by ProtonMail webclients, but our company's webclient (OBM) doesn't display accentuated character nor attachment properly.

Answer 1

Adding ;charset="utf-8" to the Content-Type header from the body part fixed my problem.
Content-Type: text/html;charset="utf-8"