Error while opening generated excel file - 'filename.xls', is in a different format than specified by the file extension

I am generating a excel file using sql query and this code -

header("Content-type: application/octet-stream");

header("Content-Disposition: attachment; filename=excelfile.xls");

But when user generates the excel and downloads it and tries to open the file, they are getting this error -

"The file you are trying to open, 'filename.xls', is in a different format     
 than specified by the file extension. Verify that the file is not corrupted 
 and is from a trusted source before opening the file. Do you want to open 
 the file now?"

and on clicking on Yes the file is opening without any problem.

How can I get rid of the problem without bothering the user ?

I have read other threads in this forum but they are unclear or do not seem to work.

Like changing extension to xlsx does not work

Can anyone help ?

You can download sample of excel generated file from here