There are 3 different forms that can be controlled in the Extensions field.

Image Extensions:
Video Extensions:
Other Extensions:

To add an extension type in the letters of the extension into the related type. If needed add a comma to seperate the extensions.

Example: You wanted to add wmv into the streaming extension.
Current extensions are:
mpeg,mpg,avi,mov,asf,rm,flv,mp3,mp4

To add wmv, that list would now look like:
mpeg,mpg,avi,mov,asf,rm,flv,mp3,mp4,wmv
This is the step you would take for any of the extension types.

Each of these types output in different ways.
The first extension acts an image, when a user uploads an image, it will display relevant hotlinking information to enable a user to post these images outside of the script.
Other options are that it will allow you to view your uploaded image.

Addin an extension to the Video types, allows Videos to streamed after they have been uploaded.

The last extension is Other, this will treat any extension, as an extension that can be downloaded, but not streamed, or viewed.

Another Option is Forbidden Extensions, this will be phased out of this version, but it will allow certain extensions to not be available to be uploaded, for security purposes.