With regards to Excel, many "in the wild" Excel workbooks will contain multiple worksheets, so it's not really practical to export a CSV for each one. Would be worth considering doing this well and supporting workbooks with multiple worksheets (same applies to Libre Office as well).
Can I also suggest you let folks play around without having to create an account (I abandoned at that point so haven't been able to play around), or at least provide a demo account that mops up stuff older than 24 hours.
