/usr/share/kconf_update/messageviewer.upd is in kf5-messagelib-data 4:17.12.3-0ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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Id=messageviewer_move_gravatar
File=kmail2rc
Group=Reader,Gravatar
Key=GravatarSupportEnabled,GravatarSupportEnabled
Key=GravatarUseDefaultImage,GravatarUseDefaultImage
Key=GravatarCacheSize,GravatarCacheSize
Key=FallbackToGravatar,FallbackToGravatar
Key=LibravatarSupportEnabled,LibravatarSupportEnabled
Key=GravatarHttpsSupport,GravatarHttpsSupport
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