<?
header("content-type: $mime_type");
header("content-disposition: attachment; filename=$file_name");
header("cache-control: private");
header("pragma: private");
echo $file_data;
?>

This set of headers works for me when I output file data from the database for the user to save or open.

Prompt user to open or save when delivering file data stored in database with PHP

http, php, file, download

PHP