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

chrome/browser/extensions/webstore_installer.cc

 // 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.
 
 #include "chrome/browser/extensions/webstore_installer.h"
 
 #include "base/basictypes.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/file_util.h"
@@ skipping 197 matching lines @@
   DownloadSaveInfo save_info;
   save_info.file_path = file;
 
   // The download url for the given extension is contained in |download_url_|.
   // We will navigate the current tab to this url to start the download. The
   // download system will then pass the crx to the CrxInstaller.
   download_util::RecordDownloadCount(
         download_util::INITIATED_BY_WEBSTORE_INSTALLER_COUNT);
   profile_->GetDownloadManager()->DownloadUrl(
       download_url_, referrer, "",
-      false, save_info, controller_->GetWebContents());
+      false, -1, save_info, controller_->GetWebContents());
 }
 
 void WebstoreInstaller::ReportFailure(const std::string& error) {
   if (delegate_)
     delegate_->OnExtensionInstallFailure(id_, error);
 
   Release();  // Balanced in Start().
 }
 
 void WebstoreInstaller::ReportSuccess() {
   if (delegate_)
     delegate_->OnExtensionInstallSuccess(id_);
 
   Release();  // Balanced in Start().
 }