When uploading large files as attachments over a poor network connection to a SP List item (using an Attachments Control in an Edit Form), the upload may time out (after approx 2 minutes) but no error is thrown. The Edit Form fires the OnSuccess Event no matter if the upload has completed or not. If not completed, the attachment is simply not there.
It would be nice to have a mechanism that catches this error so it could then trigger a pop-up message to the user saying e.g. that the network quality is not good enough to upload a file of that size.