Here's the skinny on how uploaded files are processed (from the Overview
RadAsyncUpload relies on saving temporary files for it to work properly. When posted, files are saved to the designated temp folder (App_Data/RadUploadTemp by default) with unique names. Once a postback occurs, the RadAsyncUpload fires the OnFileUploaded event for each file. The target file is passed as part of the arguments to the event and can be set as either valid (default) or invalid. After the events fire, all files marked as valid are automatically saved to the TargetFolder if it’s set.
Finally, all processed temporary files are deleted. Temporary files are also deleted after a set amount of time defined by the TemporaryFileExpiration property.
So, if you do not have the TargetFolder property set, the files remain as temporary and you can set the above property to clear them regularly. When a valid upload occurs the temp file is deleted and rewritten to the target folder location specified.
Hope this helps!