[Export to Excel 2013]Popup when open exported file.

I have been using "Export to excel 2013" plugin from this link:  https://github.com/bcit-ci/CodeIgniter/w...Excel-2013.
The exported file can be normally opened but the popup as below appears.
   

So sorry because it is Japanese but the meaning is something like "File xxx.xls's format and extension does not match.Whether the file is corrupted, it might not be safe. If the publisher can not be trusted, please do not open this file. Do you want to open a file?"

Can we turn off this popup?
And if I want to export .csv (not .xls), how to set the header?

Thanks and regards!