We do this (well, with Azure Blob Storage instead of S3, but it's the same). It works because we can have our customer execute[#] some code that authenticates t

We do this (well, with Azure Blob Storage instead of S3, but it's the same). It works because we can have our customer execute[#] some code that authenticates them, queries our server for the signature, then slices up the file and uploads it, then notifies our server when it's done. [#] including JavaScript in the page. Obviously, turning that into an automated process that runs from a Windows Scheduled Task (or worse) is not as easy.