Save As for content retrieved via POST works in most circumstances.

content/browser/tab_contents/tab_contents.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_TAB_CONTENTS_TAB_CONTENTS_H_
 #define CONTENT_BROWSER_TAB_CONTENTS_TAB_CONTENTS_H_
 #pragma once
 
 #include <deque>
 #include <map>
@@ skipping 478 matching lines @@
 
   // Misc non-view stuff -------------------------------------------------------
 
   // Helper functions for sending notifications.
   void NotifySwapped();
   void NotifyConnected();
   void NotifyDisconnected();
 
   void SetEncoding(const std::string& encoding);
 
+  // Save a URL to the local filesystem.
+  void SaveURL(const GURL& url, const GURL& referrer);
+
   // Stores random bits of data for others to associate with this object.
   // WARNING: this needs to be deleted after NavigationController.
   base::PropertyBag property_bag_;
 
   // Data for core operation ---------------------------------------------------
 
   // Delegate for notifying our owner about stuff. Not owned by us.
   content::WebContentsDelegate* delegate_;
 
   // Handles the back/forward list and loading.
@@ skipping 131 matching lines @@
   // (full-page plugins for now only) permissions.
   int content_restrictions_;
 
   // Our view type. Default is VIEW_TYPE_TAB_CONTENTS.
   content::ViewType view_type_;
 
   DISALLOW_COPY_AND_ASSIGN(TabContents);
 };
 
 #endif  // CONTENT_BROWSER_TAB_CONTENTS_TAB_CONTENTS_H_