add ppm and png images to the list

Reference: /n/sources/patch/applied/webfs-plumb-png
Date: Sun Aug 13 17:57:11 CES 2006
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/webfs/plumb.c	Sun Aug 13 17:56:37 2006
+++ /sys/src/cmd/webfs/plumb.c	Sun Aug 13 17:56:33 2006
@@ -101,6 +101,8 @@
 	{ "application/rtf", "rtf" },
 	{ "image/gif", "gif" },
 	{ "image/jpeg", "jpg" },
+	{ "image/png", "png" },
+	{ "image/ppm", "ppm" },
 	{ "image/tiff", "tiff" },
 	{ "text/html", "html" },
 	{ "text/plain", "txt" },