Add png.
 [rsc] --rw-rw-r-- M 240578 glenda sys 44584 Jan 10 11:52 sys/src/cmd/upas/ned/nedmail.c
	/n/sourcesdump/2006/0110/plan9/sys/src/cmd/upas/ned/nedmail.c:64,69 - 
	/n/sourcesdump/2006/0111/plan9/sys/src/cmd/upas/ned/nedmail.c:64,70
	  	{ "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	},