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We will be adding some additional file extensions to the BlockedFileTypes list. The BlockedFileTypes parameter specifies a list of attachment file types (file extensions) that can't be saved locally or viewed from Outlook on the web.
This change will roll out in early April and will be complete by the end of April.
This message is associated with Microsoft 365 Roadmap ID 55100.
For more information, including a complete list of extensions that will be blocked, please see https://techcommunity.microsoft.com/t5/Exchange-Team-Blog/Changes-to-File-Types-Blocked-in-Outlook-on-the-web/ba-p/874451
[How does this affect me?]
If your users are receiving affected attachments, they will report that they are no longer able to download them from Outlook on the Web. The newly blocked file types are rarely used, so most organizations will not be affected by the change. If your users are affected, you should first ensure that the application software used to open the required file type is up-to-date (in the case where files are opened by application software) or ensure that your users are familiar with the risks associated with the file types (in the case of files that are interpreted by scripting software).
[What do I need to do to prepare for this change?]
In order to minimize disruption from this change, we will not add a file extension to a policy's BlockedFileTypes list if that extension is already present in the AllowedFileTypes list. If you know that your users require access to any affected file type, you can add those file extensions to the AllowedFileTypes property of your users' OwaMailboxPolicy objects.
You can also remove the file extensions that your organization requires from the affected users' OwaMailboxPolicy configuration after the change rolls out.
For more information, including PowerShell examples, and a complete list of extensions that will be blocked, please see the Additional Information.
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