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

content/test/mock_download_manager.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_MOCK_DOWNLOAD_MANAGER_H_
 #define CONTENT_BROWSER_DOWNLOAD_MOCK_DOWNLOAD_MANAGER_H_
 #pragma once
 
 #include "content/browser/download/download_request_handle.h"
 #include "content/browser/download/download_types.h"
@@ skipping 33 matching lines @@
                                            int64 size,
                                            const std::string& hash_state,
                                            InterruptReason reason));
   MOCK_METHOD3(OnDownloadRenamedToFinalName, void(int download_id,
                                                   const FilePath& full_path,
                                                   int uniquifier));
   MOCK_METHOD2(RemoveDownloadsBetween, int(base::Time remove_begin,
                                            base::Time remove_end));
   MOCK_METHOD1(RemoveDownloads, int(base::Time remove_begin));
   MOCK_METHOD0(RemoveAllDownloads, int());
-  MOCK_METHOD6(DownloadUrl, void(const GURL& url,
+  MOCK_METHOD7(DownloadUrl, void(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));
   MOCK_METHOD1(AddObserver, void(Observer* observer));
   MOCK_METHOD1(RemoveObserver, void(Observer* observer));
   MOCK_METHOD1(OnPersistentStoreQueryComplete, void(
       std::vector<DownloadPersistentStoreInfo>* entries));
   MOCK_METHOD2(OnItemAddedToPersistentStore, void(int32 download_id,
                                                   int64 db_handle));
   MOCK_CONST_METHOD0(InProgressCount, int());
   MOCK_CONST_METHOD0(GetBrowserContext, content::BrowserContext*());
@@ skipping 20 matching lines @@
       content::DownloadManagerDelegate* delegate));
   MOCK_METHOD2(ContinueDownloadWithPath, void(content::DownloadItem* download,
                                               const FilePath& chosen_file));
   MOCK_METHOD1(GetActiveDownload, content::DownloadItem*(int32 download_id));
   MOCK_METHOD1(SetFileManager, void(DownloadFileManager* file_manager));
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_MOCK_DOWNLOAD_MANAGER_H_