User Configuration > Administrative Templates > Microsoft Office 2016 > Web Archives
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HKCU\software\policies\microsoft\office\16.0\common\internet
"Use 8 bit content-transfer-encoding": Use a content-transfer-encoding of 8bit for all parts in a Web Archive file. | "Use 8 bit only for encoding text parts": Use a content-transfer-encoding of 8bit only for text parts. | "Use RFC-approved encoding": Always use RFC-approved encodings.