--- a/dom/plugins/ipc/PluginModuleChild.cpp
+++ b/dom/plugins/ipc/PluginModuleChild.cpp
@@ -1199,16 +1199,20 @@ NPError
          const char* aTarget,
          uint32_t aLength,
          const char* aBuffer,
          NPBool aIsFile)
 {
     PLUGIN_LOG_DEBUG_FUNCTION;
     ENSURE_PLUGIN_THREAD(NPERR_INVALID_PARAM);
 
+    if (aIsFile) {
+      PLUGIN_LOG_DEBUG(("NPN_PostURL with file=true is no longer supported"));
+      return NPERR_GENERIC_ERROR;
+    }
     NPError err;
     // FIXME what should happen when |aBuffer| is null?
     InstCast(aNPP)->CallNPN_PostURL(NullableString(aRelativeURL),
                                     NullableString(aTarget),
                                     nsDependentCString(aBuffer, aLength),
                                     aIsFile, &err);
     return err;
 }

--- a/dom/plugins/test/mochitest/test_streamNotify.html
+++ b/dom/plugins/test/mochitest/test_streamNotify.html
@@ -78,12 +78,14 @@
                       is(t, "World", "GET data: URI correct");
                       testDone();
                     }, null, true), "streamTest GET data: URI");
 
     ok(!p.streamTest("data:malformed?", false, null, null,
                      function(r, t) {
                        todo(false, "Shouldn't get callback for invalid data: URI");
                      }, null, true), "streamTest GET bad data: URI");
+
+    ok(!p.postFileToURLTest("post.sjs"), "postFileToURLTest POST a file");
   }
   </script>
 
   <embed id="plugin1" type="application/x-test" width="400" height="400"></embed>