EFT GENERATED FILE FORMAT

Dear All

I have tried to generate EFT file in excel by indicating a file extension of .xlsx, however when a tried to open the file I get this message " Excel cannot open the file 'Payment CHECKBOOK 20160217-0001.xlsx' because the file format or extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file"

Is there something am doing wrong or that GP 2015 cannot generate the EFT file in ecxel.

Will appreciate your response.

Thank you.