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

content/browser/download/drag_download_file.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 "content/browser/download/drag_download_file.h"
 
 #include "base/bind.h"
 #include "base/file_util.h"
 #include "base/message_loop.h"
 #include "content/browser/download/download_stats.h"
@@ skipping 119 matching lines @@
   download_manager_->AddObserver(this);
 
   DownloadSaveInfo save_info;
   save_info.file_path = file_path_;
   save_info.file_stream = file_stream_;
 
   download_manager_->DownloadUrl(url_,
                                  referrer_,
                                  referrer_encoding_,
                                  false,
+                                 -1,
                                  save_info,
                                  web_contents_);
   download_stats::RecordDownloadCount(
       download_stats::INITIATED_BY_DRAG_N_DROP_COUNT);
 }
 
 void DragDownloadFile::DownloadCompleted(bool is_successful) {
 #if defined(OS_WIN)
   // If not in drag-and-drop thread, defer the running to it.
   if (drag_message_loop_ != MessageLoop::current()) {
@@ skipping 81 matching lines @@
 
 void DragDownloadFile::QuitNestedMessageLoop() {
   AssertCurrentlyOnDragThread();
 
   if (is_running_nested_message_loop_) {
     is_running_nested_message_loop_ = false;
     MessageLoop::current()->Quit();
   }
 }
 #endif