I'm seeing this on the server-side, which tells me the session's getting timed-out on the client-side:
15-10-2015 18:03:21.358 [http-nio-8443-exec-33] ERROR gwtupload.server.UploadLogger$Log4jLogger - UPLOAD-SERVLET (95FF435BCAAA9F0FACCD8D088603D3BB) Unexpected Exception -> Processing of multipart/form-data request failed. Unexpected EOF read on the socket
I must be missing something in the docs which explains how to reset the timeout.