I am adding a new mimetype for mp4 files to Tridion. I have followed the instructions on the Tridion Live Content site here. (login first).

And I have confirmed my settings match the blog post here.

And then I flushed my browser cache and even restarted the CMS server. But, I still get the following error:

The file extension of the selected item is unknown. The specified file does not match a supported Multimedia Type.

Any ideas?