Problem Description:

When we export Spreadsheet's book model as a Excel 2007 file with charts, the file cannot be read by Excel 2007. When opening the corrupt file in Excel, it shows a message Excel found unreadable content in 'unread.xlsx'. Do you want to recover the content of this workbook? If you trust the source of this workbook, click yes.

Reproducing Step:

1. run Spreadsheet (or use online demo)
2. insert a line chart
3. export it as a Excel file
4. open the exported Excel file with Excel 2007

Expected:
the exported file is open correctly.

Actual:
Excel shows the error message mentioned in "Problem Description". If we choose to recover the content, the exported file can be open but charts disappear.