Send HTTP security headers (CSP, etc) on file downloads

File downloads should return HTTP security headers as another layer of
protection against someone framing a MantisBT file_download link to a
file with a harmful MIME type such as text/html.

file_download.php

@@ -129,6 +129,8 @@
 	ini_set( 'zlib.output_compression', false );
 }
 
+http_security_headers();
+
 # Make sure that IE can download the attachments under https.
 header( 'Pragma: public' );