gitweb: Fix usability of $prevent_xss

With XSS prevention on (enabled using $prevent_xss), blobs
('blob_plain') of all types except a few known safe ones are served
with "Content-Disposition: attachment".  However the check was too
strict; it didn't take into account optional parameter attributes,

  media-type     = type "/" subtype *( ";" parameter )

as described in RFC 2616

  http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.17
  http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.7

This fixes that, and it for example treats following as safe MIME
media type:

  text/plain; charset=utf-8

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -4752,7 +4752,7 @@ sub git_blob_plain {
 	# want to be sure not to break that by serving the image as an
 	# attachment (though Firefox 3 doesn't seem to care).
 	my $sandbox = $prevent_xss &&
-		$type !~ m!^(?:text/plain|image/(?:gif|png|jpeg))$!;
+		$type !~ m!^(?:text/plain|image/(?:gif|png|jpeg))(?:[ ;]|$)!;
 
 	print $cgi->header(
 		-type => $type,