add a table entry to display .png files.

Reference: /n/sources/patch/applied/nedmail-png-display
Date: Mon Jan  9 09:09:46 CET 2006

--- /sys/src/cmd/upas/ned/nedmail.c	Mon Jan  9 09:09:36 2006
+++ /sys/src/cmd/upas/ned/nedmail.c	Mon Jan  9 09:09:30 2006
@@ -64,6 +64,7 @@
 	{ "image/bmp",			"bmp",	0,	"image"	},
 	{ "image/jpeg",			"jpg",	0,	"image"	},
 	{ "image/gif",			"gif",	0,	"image"	},
+	{ "image/png",			"png",	0,	"image"	},
 	{ "application/pdf",		"pdf",	0,	"postscript"	},
 	{ "application/postscript",	"ps",	0,	"postscript"	},
 	{ "application/",		0,	0,	0	},