Firefox adds xls extension when downloading excel files from problem

My problem is this.
I have this excel report file that I crafted using PHPExcel for my php application. I don’t have any problem accessing the file from my ubuntu machine with major browsers. But from my windows box, FF will add this .xls extension on top of my file name [myreport.xlsx becomes myreport.xlsx.xls] and this would confuse windows on how to create and will finally decide to open it as binary gibberish texts.
The solution is to assign the correct MIME type for the xlsx extension. I got that gem from here

application/vnd.openxmlformats-officedocument.spreadsheetml.sheet

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