This function can return a false positive if you set a MAX_FILE_SIZE limit in the HTML form sending the file and then proceeed to send a file bigger than this max.  If a call is made to _is_uploaded_file to see if the file made it to the server, the function will return TRUE even though the file doesn't exist on the server and the filename is set to "none".  In this case, the function _is_uploaded_file should return FALSE, not TRUE because it has not been uploaded.