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When you download the file from MaxMind it's a .mmdb.gz file. That's a compressed file. You will need to uncompress it using 7-Zip (Windows), Archiver (macOS) or gunzip (Linux). Then upload it. The file must be in its uncompressed form for MaxMind's library to be able to read it.

I forgot to also note that the warning is issued based on the modification date of the file. If it's older than a month from the server's current date and time you get a warning. So the other thing you must check is that the modification date of the uploaded file is today and that your server's date and time are correctly set up.

I was typing a similar response, assuming you're on a Linux server.

On a Windows server the FTP uploads use a local user account, not an IIS (IUSR*) user account. Namely, they use the same user account you use to connect to FTP with. By default, the uploaded files only get full access privileges for the owner and read privileges for the Users group. However, the IUSR* accounts do not belong to the Users group, therefore making the file unreadable and unwriteable to the web server (since PHP runs either as a SAPI module or through FastCGI under the same IUSR* account as your site).

My observations are on IIS 7 on Windows 10 but it shouldn't be that far removed from Windows Server 2012. IIRC this behavior exists since at least Windows Vista which predates Windows Server 2012.