您尝试上传的文件类型是不允许的。在codeigniter

I have problem in upload files using codeigniter. I only set the file type to xls only for excel 2003 format only. In some computers uploading works perfectly but in some computers the problem exist "The filetype you are attempting to upload is not allowed". I don't know why this problem occur since it's working in other computers.I have problem in upload files using codeignite