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

content/public/browser/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.
 //
 // The DownloadManager object manages the process of downloading, including
 // updates to the history system and providing the information for displaying
 // the downloads view in the Destinations tab. There is one DownloadManager per
 // active browser context in Chrome.
 //
 // Download observers:
@@ skipping 150 matching lines @@
   // deleted is returned back to the caller.
   virtual int RemoveDownloads(base::Time remove_begin) = 0;
 
   // Remove all downloads will delete all downloads. The number of downloads
   // deleted is returned back to the caller.
   virtual int RemoveAllDownloads() = 0;
 
   // Downloads the content at |url|. |referrer| and |referrer_encoding| are the
   // referrer for the download, and may be empty. If |prefer_cache| is true,
   // then if the response to |url| is in the HTTP cache it will be used without
-  // revalidation. |save_info| specifies where the downloaded file should be
+  // revalidation. If |post_id| is non-negative, then it identifies the post
+  // transaction used to originally retrieve the |url| resource - it also
+  // requires |prefer_cache| to be |true| since re-post'ing is not done.
+  // |save_info| specifies where the downloaded file should be
   // saved, and whether the user should be prompted about the download.
   // |web_contents| is the web page that the download is done in context of,
   // and must be non-NULL.
   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) = 0;
 
   // Allow objects to observe the download creation process.
   virtual void AddObserver(Observer* observer) = 0;
 
   // Remove a download observer from ourself.
   virtual void RemoveObserver(Observer* observer) = 0;
 
   // Called by the embedder, after creating the download manager, to let it know
@@ skipping 85 matching lines @@
   friend class base::RefCountedThreadSafe<
       DownloadManager, content::BrowserThread::DeleteOnUIThread>;
   friend struct content::BrowserThread::DeleteOnThread<
       content::BrowserThread::UI>;
   friend class base::DeleteHelper<DownloadManager>;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_DOWNLOAD_MANAGER_H_