How to limit file extension of attachments?

[Migrated From Jforum.net]

Hi, all,
I had setup JForum 2.1.4 successfully on my web site. There is one more question left, I don't know how to limit file extension of attachments. For example, I only want the user to attach .zip/.rar/.tar.gz files as attachments to save my space. So before they attach a file, they must compress it first.

How to do it? Any help is appreciated.
[originally posted on jforum.net by David Lee]