[imp] filename becomes ???.txt if not English of .txt

[Nethub Online - Ming]

Hi all,

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    5.2.5

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I tested that if I compose new email and attach a file with non English
characters (e.g.: めまして.txt / 測試.txt) , the attachment filename will be
changed to ???.txt

This only happen for .txt file, but no problem for .png

Even I just save it in draft, it is also ???.txt

Below is part of mail source, you can see .png is using utf-8 but .txt is
using us-ascii, I think it is the core problem. Is there anyone can help?

--=_niDwCrm6H6ppSnSrbXnJVqo
Content-Type: text/plain; name*=us-ascii''%3F%3F%3F%3F.txt
Content-Disposition: attachment; size=10;
 filename*=us-ascii''%3F%3F%3F%3F.txt

fjskafjsal
--=_niDwCrm6H6ppSnSrbXnJVqo
Content-Type: image/png;
 name*=utf-8''%E3%82%81%E3%81%BE%E3%81%97%E3%81%A6.png
Content-Disposition: attachment; size=7736;
 filename*=utf-8''%E3%82%81%E3%81%BE%E3%81%97%E3%81%A6.png
Content-Transfer-Encoding: base64