Based on this
forum thread, The temporary filename is not available on the client due to security reasons. The server side code you tried does not return the Temporary file name. In that code Server.MapPath(targetFolder + "/" + e.File.FileName) statement provides the final path of the uploaded file on the server.
Finally, all processed temporary files are deleted. Temporary files are also deleted after a set amount of time
defined by the TemporaryFileExpiration property. If this property is not set, it will be deleted automatically by the .NET framework after 4 hours.