I'm not commenting on the issue in general, as things got a bit edgy, however, my .02:
I use App Engine, or other cloud products, as they make things easy for me, for multi-part uploads, my expectancy is having wrapper libraries that make multi-part uploads dead-simple (JS, Mobile: iOS, Java, Cordova, possibly others)
In fact, while you are at it, it could even be a large/inclusive library for all high level things combined into one package, and possibly, one could hand-pick certain things and get a smaller library if size is a concern
One example was the old Channel JS library, while I haven't used it myself, design-wise it was a nice library, tackled a complex system in a simple way
From a cloud users' perspective, multi-part uploads is indeed a bit complex, and icky challenges like these consume a lot of time, and in the end, when you look back, you realise you spent a considerable amount of time on these kind of integrations/challenges and on a larger scale, it probably has a high impact