No se si esto vaya a influir, pero, ponle un nombre a las etiquetas del formulario.
Código HTML:
Ver original<input type="file" name="archivo" /> <input type="submit" name="submit" value="Subir" />
Nunca he usado esa librería que estas utilizando.
Edit: Veo esto en la documentacion de FileUpload:
Cita: Why is parseRequest() returning no items?
This most commonly happens when the request has already been parsed, or processed in some other way. Since the input stream has aleady been consumed by that earlier process, it is no longer available for parsing by Commons FileUpload.