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

content/browser/tab_contents/tab_contents.cc

Index: content/browser/tab_contents/tab_contents.cc
diff --git a/content/browser/tab_contents/tab_contents.cc b/content/browser/tab_contents/tab_contents.cc
index 78b80246afc2526d741901f8cbcf3104e4b931d5..bb64ed06f08cecf0fa748f81c1c94d29ec594b85 100644
--- a/content/browser/tab_contents/tab_contents.cc
+++ b/content/browser/tab_contents/tab_contents.cc
@@ -934,21 +934,10 @@ bool TabContents::IsSavable() {
 void TabContents::OnSavePage() {
   // If we can not save the page, try to download it.
   if (!IsSavable()) {
-    DownloadManager* dlm = GetBrowserContext()->GetDownloadManager();
-    const GURL& current_page_url = GetURL();
-    if (dlm && current_page_url.is_valid()) {
-      DownloadSaveInfo save_info;
-      save_info.prompt_for_save_location = true;
-      dlm->DownloadUrl(current_page_url,
-                       GURL(),
-                       "",
-                       true,  // prefer_cache
-                       save_info,
-                       this);
-      download_stats::RecordDownloadCount(
-          download_stats::INITIATED_BY_SAVE_PACKAGE_FAILURE_COUNT);
-      return;
-    }
+    SaveURL(GetURL(), GURL());
+    download_stats::RecordDownloadCount(
+        download_stats::INITIATED_BY_SAVE_PACKAGE_FAILURE_COUNT);
+    return;
   }
 
   Stop();
@@ -1425,10 +1414,7 @@ void TabContents::OnUpdateZoomLimits(int minimum_percent,
 }
 
 void TabContents::OnSaveURL(const GURL& url) {

[Randy Smith (Not in Mondays), 2012/02/02 19:46:10]

When does this get called?  I ask because we should have a UMA for it.  I'm
doing a revamp of the initiation UMAs at the moment, so I'll just fold this in,
but I'd like to know what to call the UMA :-}.

[cbentzel, 2012/02/03 15:54:47]

This is currently called when the save button icon is clicked within the bundled
PDF viewer plugin.

-  DownloadManager* dlm = GetBrowserContext()->GetDownloadManager();
-  DownloadSaveInfo save_info;
-  save_info.prompt_for_save_location = true;
-  dlm->DownloadUrl(url, GetURL(), "", true, save_info, this);
+  SaveURL(url, GetURL());
 }
 
 void TabContents::OnEnumerateDirectory(int request_id,
@@ -2288,6 +2274,27 @@ void TabContents::SetEncoding(const std::string& encoding) {
       GetCanonicalEncodingNameByAliasName(encoding);
 }
 
+void TabContents::SaveURL(const GURL& url, const GURL& referrer) {
+  DownloadManager* dlm = GetBrowserContext()->GetDownloadManager();
+  if (!dlm)
+    return;
+  int64 post_id = -1;
+  if (url == GetURL()) {

[Randy Smith (Not in Mondays), 2012/02/02 19:46:10]

Could you comment on the purpose of this test?  (Either in the code, or to me if
it should be obvious from the code.)  It seems a bit dodgy given how the helper
function is used in this file; it's as if you're saying "If we're doing a save
from OnSavePage(), probe for the post_id, otherwise don't." in which case it
should be a level up.  But that may be because I don't understand OnSaveUrl().

[cbentzel, 2012/02/03 15:54:47]

Added a comment.

+    const NavigationEntry* entry = controller_.GetActiveEntry();
+    if (entry)
+      post_id = entry->GetPostID();
+  }
+  DownloadSaveInfo save_info;
+  save_info.prompt_for_save_location = true;
+  dlm->DownloadUrl(url,
+                   referrer,
+                   "",
+                   true,  // prefer_cache
+                   post_id,
+                   save_info,
+                   this);
+}
+
 void TabContents::CreateViewAndSetSizeForRVH(RenderViewHost* rvh) {
   RenderWidgetHostView* rwh_view = GetView()->CreateViewForWidget(rvh);
   // Can be NULL during tests.