How can I allow only one file extension?

[Migrated From Jforum.net]

Ok, I implemented this feature for version 2.1.7, and update d the documentation already, with the following statement:

If you want to deny all extensions, insert an asterisk / star ( * ) in the extension field, and JForum will handle the rest for you.

Rafael
[originally posted on jforum.net by Rafael Steil]

[Migrated From Jforum.net]

Hi, I want to disable all file extensions except one extension.
For example, I want to only allow .zip as the file extension of attachment, disable all other file extensions. How can I fulfill it?
I had read this document for 3 times, but I can not figure out how to do it.
http://www.jforum.net/confluence/display/admin/Extension+Groups
[originally posted on jforum.net by David Lee]