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Most browsers suppor dragging files on top of <input type="file">, so I guess you could use a normal upload component in non-immediate mode and some CSS to make it look like a "dropzone". The trigger the upload from a normal button click listener.
From usability POV, I'd really suggest to let the upload just start right away. It just doesn't make sense to make your users wait for the upload to happen, any more than required. If you want to save some memory, just stream it to a file and use from to server side when you actually need it.