Content-type в заголовках
header("Content-type:application/vnd.ms-word");
header("Content-Disposition:attachment;filename=test.doc");
header("Content-type:application/vnd.ms-excel");
header("Content-Disposition:attachment;filename=test.xls");
header("Content-type:application/vnd.ms-PowerPoint");
header("Content-Disposition:attachment;filename=test.ppt");