From Changes to File Types Blocked in Outlook on the Web:
Microsoft’s Exchange team has updated the File Types that is blocked in Outlook Web. The list has been updated as they see it can be risky downloading them in current scenarios. These new extension has been added to the BlockedFileTypes property of existing OwaMailboxPolicy objects. This change will prevent Outlook.com users from downloading attachments that have those file extensions. Microsoft will start rolling them out in October in Exchange Online.
The Exchange announcement is here.