Looks pretty straightforward. But why use this over, for example, crate? Also, how are you going to make revenue out of it? S3 does cost money.
#edit: Almost forgot. Well done on the UI front. Minor gripe: an actual button to select the file to upload would be nice. Having to launch Finder and browsing to the file I'm looking for, then dragging it back to the browser, can take much longer than just clicking a button, browsing and clicking "open".