diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index 590bf7170e69..14c0fa1acfd0 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -133,6 +133,7 @@
 #include "nsIWebBrowserChromeFocus.h"
 
 #include "nsPluginError.h"
+#include "nsIURL.h"
 
 static NS_DEFINE_IID(kDeviceContextCID, NS_DEVICE_CONTEXT_CID);
 static NS_DEFINE_CID(kSimpleURICID, NS_SIMPLEURI_CID);
@@ -4329,6 +4330,29 @@ nsDocShell::NewContentViewerObj(const char *aContentType,
                                 nsIStreamListener ** aContentHandler,
                                 nsIContentViewer ** aViewer)
 {
+    nsCOMPtr<nsIPluginHost> pluginHost (do_GetService(kPluginManagerCID));
+    nsCOMPtr<nsIChannel> aOpenedChannel = do_QueryInterface(request);
+
+    // check plugins to see if there is an override mime type for this extension
+    if (pluginHost &&
+        NS_FAILED(pluginHost->IsPluginEnabledForType(aContentType))) {      
+      // get the extension from the url which we get from the channel
+      nsCOMPtr<nsIURI> uri;
+      if (NS_SUCCEEDED(aOpenedChannel->GetURI(getter_AddRefs(uri)))) {
+        nsCOMPtr<nsIURL> url = do_QueryInterface(uri);
+        if (url) {
+          nsCAutoString fileExtension;
+          url->GetFileExtension(fileExtension);
+          if (!fileExtension.IsEmpty()) {            
+            // ask the plugin host for a mime type for this extension
+            const char* pluginMimeType;
+            if (NS_SUCCEEDED(pluginHost->IsPluginEnabledForExtension(fileExtension.get(), pluginMimeType)))
+              aContentType = pluginMimeType;
+          }
+        }
+      }
+    }
+
     //XXX This should probably be some category thing....
     nsCAutoString contractId(NS_DOCUMENT_LOADER_FACTORY_CONTRACTID_PREFIX
                              "view"
@@ -4344,7 +4368,7 @@ nsDocShell::NewContentViewerObj(const char *aContentType,
         docLoaderFactory(do_CreateInstance(contractId.get()));
     if (!docLoaderFactory) {
         // try again after loading plugins
-        nsCOMPtr<nsIPluginManager> pluginManager = do_GetService(kPluginManagerCID);
+        nsCOMPtr<nsIPluginManager> pluginManager = do_QueryInterface(pluginHost);
         if (!pluginManager)
             return NS_ERROR_FAILURE;
 
@@ -4360,8 +4384,6 @@ nsDocShell::NewContentViewerObj(const char *aContentType,
             return NS_ERROR_FAILURE;
     }
 
-    nsCOMPtr<nsIChannel> aOpenedChannel = do_QueryInterface(request);
-
     // Now create an instance of the content viewer
     NS_ENSURE_SUCCESS(docLoaderFactory->CreateInstance("view",
                                                        aOpenedChannel,
diff --git a/modules/plugin/base/src/nsPluginHostImpl.cpp b/modules/plugin/base/src/nsPluginHostImpl.cpp
index 804af532f03e..57b0ec5e6651 100644
--- a/modules/plugin/base/src/nsPluginHostImpl.cpp
+++ b/modules/plugin/base/src/nsPluginHostImpl.cpp
@@ -3859,20 +3859,19 @@ NS_IMETHODIMP nsPluginHostImpl::TrySetUpPluginInstance(const char *aMimeType,
   if(!aURL)
     return NS_ERROR_FAILURE;
 
-  // if don't have a mimetype, check by file extension
-  if(!aMimeType)
-  {
-    char* extension;
-
-    nsCAutoString filename;
-    aURL->GetPath(filename);
-    extension = PL_strrchr(filename.get(), '.');
-    if(extension)
-      ++extension;
-    else
-      return NS_ERROR_FAILURE;
-
-    if(IsPluginEnabledForExtension(extension, mimetype) != NS_OK)
+  // if don't have a mimetype or no plugin can handle this mimetype
+  // check by file extension
+  if(!aMimeType || NS_FAILED(IsPluginEnabledForType(aMimeType))) {
+    nsCOMPtr<nsIURL> url = do_QueryInterface(aURL);
+    if (!url) return NS_ERROR_FAILURE;
+    
+    nsCAutoString fileExtension;
+    url->GetFileExtension(fileExtension);
+    
+    // if we don't have an extension or no plugin for this extension,
+    // return failure as there is nothing more we can do
+    if (fileExtension.IsEmpty() || 
+        NS_FAILED(IsPluginEnabledForExtension(fileExtension.get(), mimetype)))
       return NS_ERROR_FAILURE;
   }
   else