Hello Leo,
If you need this to be done on a particular Document Library, then you need to create a Event Receiver for the library. Check this
link to get more information.
You need to have a handler for ItemAdding(), and in this method check for the file extension.
If file is found to have a video or image extension, then cancel the event (using properties.Cancel = true), and stop the item upload.
Ram Prasad Meenavalli | MCTS SharePoint 2010 | MCPD SharePoint 2010