fix post form's content type; add support for image/png and image/ppm.

Reference: /n/sources/patch/applied/webfs-contenttype
Date: Sun Jul 23 18:36:00 CES 2006
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/webfs/http.c	Sun Jul 23 18:34:44 2006
+++ /sys/src/cmd/webfs/http.c	Sun Jul 23 18:34:25 2006
@@ -11,7 +11,7 @@
 #include "dat.h"
 #include "fns.h"
 
-char PostContentType[] = "application/octet-stream";
+char PostContentType[] = "application/x-www-form-urlencoded";
 int httpdebug;
 
 typedef struct HttpState HttpState;