Exporting tabular data in Numbers-readable format

I have app that exports data in CSV. Recently I added "Open In..." feature for opening .csv files in other apps. I would like to be able to have Numbers (on iPad) open exported files as well.

Trawling through Numbers's Info.开发者_运维知识库plist, I saw that it supports four document types:

1. com.apple.iWork.Numbers.sffnumbers
2. com.apple.iWork.Numbers.numbers
3. com.microsoft.excel.xls
4. org.openxmlformats.spreadsheetml.sheet

So, I am wondering if anyone knows where I can find specifications for one or more of these file formats or and example spreadsheet or code that would do the job. I have googled a fair bit - and I assume that formats 1-3 are proprietary. I am not sure about #4 and have found bits and pieces, but it isn't clear to me how they would fit together to constitute a readable document.

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Format 4 is the OOXML spreadsheet format. You may be able to find the references you need at the Wikipedia page.