Settings \ Organization \ Allowed Media File Extensions
Select the Allowed Media File Extensions Tab to Add or Edit and Remove Media File Extensions you don't want to make available to Users for uploading certain Media file types.
To Add a Media File Extension, click the Add button.
You may then type in the New Extension of the file you would like to Add, and click Ok.
Removing Media File Extensions will prevent those file types from being uploaded as Media in your Organization.
To Remove Media File Extensions, click Edit then click the "x" for the specific file type you would like to remove. When you are finished, click Save.