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Uploading the zipped file to dropbox and unzipping it inside would save a lot of time and traffic for users. Some of the web services already exist for that (like this one http://www.quora.com/Is-the...
I'm working on with a large .zip that eventually has to be sorted and left on the cloud anyway. Right now I have to download the .zip, then unzip it, then re-upload and sort out. Honestly, this sounds like it needs to be a basic feature since it will save your own servers the additional traffic.