Internally, RadAsyncUpload can choose between four modules for uploading - IFrame, Flash, Silverlight and FileApi. The module with higher priority is Silverlight. If there is no Silverlight installed on the client machine, RadAsyncUpload will utilize the Flash or FileAPI (depends on the browser) module. If there is no Flash as well, RadAsyncUpload will use the IFrame module which is supported out of the box on all browsers.
The FileApi standard is not supported by IE, therefore RadAsyncUpload FileApi module does not work under IE. The FileApi works under FireFox 4.0+, Safari 5.0+, Google Chrome. You don't have to do anything to implement this modules, they will automatically turn on if the underlying browser supports the given technology.
the Telerik team