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

content/browser/download/download_manager_impl.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_DOWNLOAD_DOWNLOAD_MANAGER_IMPL_H_
 #define CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_MANAGER_IMPL_H_
 #pragma once
 
 #include <map>
@@ skipping 46 matching lines @@
                                             const FilePath& full_path,
                                             int uniquifier) OVERRIDE;
   virtual int RemoveDownloadsBetween(base::Time remove_begin,
                                      base::Time remove_end) OVERRIDE;
   virtual int RemoveDownloads(base::Time remove_begin) OVERRIDE;
   virtual int RemoveAllDownloads() OVERRIDE;
   virtual void DownloadUrl(const GURL& url,
                            const GURL& referrer,
                            const std::string& referrer_encoding,
                            bool prefer_cache,
+                           int64 post_id,
                            const DownloadSaveInfo& save_info,
                            content::WebContents* web_contents) OVERRIDE;
   virtual void AddObserver(Observer* observer) OVERRIDE;
   virtual void RemoveObserver(Observer* observer) OVERRIDE;
   virtual void OnPersistentStoreQueryComplete(
       std::vector<DownloadPersistentStoreInfo>* entries) OVERRIDE;
   virtual void OnItemAddedToPersistentStore(int32 download_id,
                                             int64 db_handle) OVERRIDE;
   virtual int InProgressCount() const OVERRIDE;
   virtual content::BrowserContext* GetBrowserContext() const OVERRIDE;
@@ skipping 183 matching lines @@
   content::DownloadManagerDelegate* delegate_;
 
   // TODO(rdsmith): Remove when http://crbug.com/85408 is fixed.
   // For debugging only.
   int64 largest_db_handle_in_history_;
 
   DISALLOW_COPY_AND_ASSIGN(DownloadManagerImpl);
 };
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_MANAGER_IMPL_H_