Microsoft is banning almost 40 new types of file extensions on its Outlook email platform. The move is to protect email users from what it deems at-risk file attachments, which are typically sent with malicious scripts or executables. In total, Microsoft has now blocked 142 file extensions that it deems as at risk. Microsoft said many of these newly-blocked file extensions are rarely used, so most organizations will not be affected by the change. Microsoft will also block extensions being used by vulnerable applications, which could be used to exploit security vulnerabilities in third-party software.
Source: https://threatpost.com/microsoft-blacklists-dozens-of-new-file-extensions-in-outlook/148737/