Order Allow,Deny
Deny from all
<FilesMatch "^[^.]+\.(?:[Jj][Pp][Ee]?[Gg]|[Pp][Nn][Gg]|[Gg][Ii][Ff]|[Pp][Dd][Ff])$">
Allow from all
</FilesMatch>

or:

Order Allow,Deny
Deny from all
<FilesMatch "^[^.]+\.(?i:jpe?g|png|gif|pdf)$">
Allow from all
</FilesMatch>