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

content/browser/renderer_host/resource_dispatcher_host.cc

Index: content/browser/renderer_host/resource_dispatcher_host.cc
diff --git a/content/browser/renderer_host/resource_dispatcher_host.cc b/content/browser/renderer_host/resource_dispatcher_host.cc
index f39d36465d68a243d1284793f3302fe30ab78758..3d9006dae4d9a6ca92c8ef679af126f8dae87559 100644
--- a/content/browser/renderer_host/resource_dispatcher_host.cc
+++ b/content/browser/renderer_host/resource_dispatcher_host.cc
@@ -893,7 +893,14 @@ net::Error ResourceDispatcherHost::BeginDownload(
   request->set_context(request_context);
   int extra_load_flags = net::LOAD_IS_DOWNLOAD;
   if (prefer_cache) {
-    extra_load_flags |= net::LOAD_PREFERRING_CACHE;
+    // If there is upload data attached, only retrieve from cache because there
+    // is no current mechanism to prompt the user for their consent for a
+    // re-post. For GETs, try to retrieve data from the cache and skip
+    // validating the entry if present.
+    if (request->get_upload() != NULL)
+      extra_load_flags |= net::LOAD_ONLY_FROM_CACHE;
+    else
+      extra_load_flags |= net::LOAD_PREFERRING_CACHE;
   } else {
     extra_load_flags |= net::LOAD_DISABLE_CACHE;
   }