@NigDig This Bug is fixed with Patch “B” and also in the upcoming MR3 version.
My colleague is at Kaspersky Internet security version 21.2.16.590 (b). I guess this means patch “B” is applied and “Mail antivirus” is still changing email attachment extensions in Mozilla Thunderbird. I had to manually disable it for all “safe” file types.
Microsoft Outlook 2019 and Attachments, all attachments come with a weird extension_, .xls
in Kaspersky Internet Security
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My colleague is at Kaspersky Internet security version 21.2.16.590 (b). I guess this means patch “B” is applied and “Mail antivirus” is still changing email attachment extensions in Mozilla Thunderbird. I had to manually disable it for all “safe” file types.