PHP - Read-Only spreadsheet filetype?

Question

I'm using a simple web-based PHP application that outputs a table as a spreadsheet

header("Content-Disposition: attachment; filename=" . $filename . ".xls");
header("Content-Type: application/vnd.ms-excel");

//inserts tab delimited text

But I'm finding the downloaded spreadsheet opens as a read-only file and must be saved locally and (in Excel on Windows) the type changed to XLS (from HTML). Is there a way to set the attribute of filetype correctly so that doing a simple save doesn't require correcting the filetype?

Is the file downloaded read-only by nature of security or is this not normal?

Also I don't like the automatic borders created when opening the spreadsheet in Excel or OpenOffice (on Linux). I would prefer to have no border formatting. Is there a way in the file to specify no added formatting or is this built into those applications?