iCab Mobile may work, but you have to copy (Open In) the file to the app first.
Safari can not access files stored in other apps, and does not have it's own file space. This is why the Choose File and Upload buttons are grayed out. iCab Moble has file space that you can upload from, once the file is there; however it does not work with all sites. Some sites depend on browser extentions that are not supported by the iPad.