You need PostBack to upload the files- here is how RadAsyncUpload works as it is described in its documentation
: RadAsyncUpload relies on saving temporary files to work. 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
Hope this information will explain the issue.
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