Sharepoint - What are the client side requirements to enable the "upload multiple files" link?

Here are the requirements for 'upload multiple files'

1. The 32 bit version of Internet Explorer. The 64 bit version does not support it and neither do other, non IE, browsers.
2. The STSUpld.UploadCtl IE add-in must be enabled.
3. Office 2003 or newer on the client
4. Client integration will need to be enabled for the web app.

For details see this post.