[content shell] further restrict what files to open during layout tests

content/shell/shell_download_manager_delegate.cc

Index: content/shell/shell_download_manager_delegate.cc
diff --git a/content/shell/shell_download_manager_delegate.cc b/content/shell/shell_download_manager_delegate.cc
index e0ed2617df1aa5154f8a8d05dbaf23ed49f0f387..dc5a87da1594545088be4bb9d642aa93c2ad06ab 100644
--- a/content/shell/shell_download_manager_delegate.cc
+++ b/content/shell/shell_download_manager_delegate.cc
@@ -91,7 +91,8 @@ bool ShellDownloadManagerDelegate::ShouldOpenDownload(
       DownloadItem* item,
       const DownloadOpenDelayedCallback& callback) {
   if (CommandLine::ForCurrentProcess()->HasSwitch(switches::kDumpRenderTree) &&
-      WebKitTestController::Get()->IsMainWindow(item->GetWebContents())) {
+      WebKitTestController::Get()->IsMainWindow(item->GetWebContents()) &&
+      item->GetMimeType() == "text/html") {
     WebKitTestController::Get()->OpenURL(
         net::FilePathToFileURL(item->GetFullPath()));
   }