Microsoft blocked 38 file extensions in Outlook web to prevent users from downloading attachments with those files extensions. Microsoft said that the newly blocked file types are rarely used, so most organizations will not be affected by the change. Previously Outlook blocked 104 file types, now Microsoft expanded it by adding additional 38 File types to enhance the security of Outlook web. Users can Whitelist any blocked attachments using Microsoft Exchange Server account, this can be done by contacting the administrator to make the changes in security.”]
Source: https://gbhackers.com/blocked-extensions-outlook-web/