Encrypted xlsx file won't open and prompt for password. File extension does not match

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Occasional Visitor
One of our external contacts have started using Office 365 encryption on their emails. So they send us an email, we open the "attachment" which takes us to a log in page for Office 365 encryption and we gain access using a one-time pass code. No problem. We can then read the email and see the xlsx attachments and download them, no problem. The external contact has advised the password to open the attachments. When we try to open the downloaded attachments, Excel 2016 tells us it can't open the file as the file extension doesn't match the file type or the file may be corrupted. It never gives us the option to enter the password. It's driving us mad! Last month a colleague of mine who has exactly the same setup managed it, but he has now left the company. Our IT department, the external company, and my other colleagues all cannot figure out the problem. A few of my team have tried and had the same issues. My IT department have even tried using Excel 365 rather than Excel 2016, but still no luck.

It seems it should be an easy fix but I'm at a loss!
1 Reply
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Not sure to understand...

Is the xlsx file protected by a password and sent as an attachment of an encrypted message too?

If this is the case, has your external contact tried to send a password-protected test xlsx file attached to a plain message? Does it work?