Microsoft has banned 38 new file extensions from Outlook online, bringing the total number of forbidden file types to 104.
The company hasn't said exactly when the change will roll out, but it's expected to come into force very soon. When it does, you will no longer be able to download files with the blacklisted extensions unless your system admin has made a specific exception.
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As ZDNet reports, the newly blocked file types are often used to send malicious content, but are rarely used legitimately, so Microsoft doesn't expect the update to cause much trouble for Outlook users.
The new blacklist includes Java, PowerShell and Python files, plus digital certificates and files that could be used to exploit security vulnerabilities in third-party software. Many of those vulnerabilities have now been patched, but Microsoft has decided to block them anyway for the benefit of anyone running outdated software. For more details, check out Microsoft's full list of all banned file types.
If you do happen to use any of these file extensions, Microsoft has also put together a guide explaining how to whitelist any that you need.