After you've opened the file in iCab, the file is in the app's folder system. You then visit the website as usual and choose the upload options. It should take you to a menu/window where you can browse for the file in iCab and upload it, much like you would have on a computer.
iCab Mobile uploading works with most sites, but here are exceptions. When ti does not work, it doesn't, and there isn't much you can do about it.