Export to excel in IE9 doesn't work

I got the proxyURL and the forceProxy=true but when opening the file (.xlsx) I get the error "We found a problem with some content in "test.xlsx". Do you want to try to recover as much as we can? If you trust the source of the this workbook, click Yes".

So I opened the xlsx and found the problem is the sheet1.xml is actually renamed as sheet1.00.xml which makes Excel not recognize and throw the error (if I rename it to sheet1.xml the file opens fine).

This only happens in IE9. IE10+ and other browsers (tried the last versions of FF and Chrome) everything works fine, the file comes named as sheet1.xml and the xlsx opens correctly.

Is this a bug? Can I do something to get this to work on IE9?

Thanks