it was based xml, also can be opened in ie or any text editor. support this format isn't a difficult thing.
Code:
<html xmlns:v="urn:schemas-microsoft-com:vml"
xmlns:o="urn:schemas-microsoft-com:office:office"
xmlns:x="urn:schemas-microsoft-com:office:excel"
xmlns="http://www.w3.org/TR/REC-html40">
<head>
<meta http-equiv=Content-Type content="text/html; charset=us-ascii">
<meta name=ProgId content=Excel.Sheet>
<meta name=Generator content="Microsoft Excel 11">
<link rel=File-List href="result.files/filelist.xml">
<link rel=Edit-Time-Data href="result.files/editdata.mso">
<link rel=OLE-Object-Data href="result.files/oledata.mso">
open it in a text editor, we can know this file is a Microsoft Excel 11 format.