[issue39771] EmailMessage.add_header doesn't work

[hwgdb Smith]

hwgdb Smith <hwgdb123 at gmail.com> added the comment:

"but msg.as_string() doesn't change. " , I mean using 

  filename=file_name  
or
  filename=("GBK", "", f"{file_name}")
or
  filename=("utf-8", "", f"{file_name}")

msg.as_string() doesn't change.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39771>
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