From gbhackers.com
Microsoft blocked 38 file extensions in Outlook web to prevent user’s from downloading attachments with those files extensions.
Outlook contains built-in spam mail filter which prevents the junk mail getting into the inbox. Now Microsoft added 38 file extensions to BlockedFileTypes property of existing OwaMailboxPolicy objects.
Previously outlook blocked 104 file types, now Microsoft expanded it by adding additional 38 File types to enhance the security.