It seems to be caused by mail client parser problem.
HigLabo encode filename by RFC2231 but many client could not parse it correctly.
Gmail and outlook.com could not RFC2231.
GMail, Outlook.com encode filename like below.
Content-Disposition: attachment; filename="=?ISO-2022-JP?B?GyRCJy8nYCdTJ20nWxsoQiAbJEInZCdWJ1wnYydkJ2AnUydtJ1sbKEIgGyRCJ1UnYCdcJ2UnXidWJ18nZBsoQi50eHQ=?="
This encoding is not correct in RFC.But it is used as old style and many mail client parse it correctly.
I'll add a feature that you can select how to encode filename in next release.