Move download filename determintion into a separate class.

chrome/browser/download/download_prefs.cc

Index: chrome/browser/download/download_prefs.cc
diff --git a/chrome/browser/download/download_prefs.cc b/chrome/browser/download/download_prefs.cc
index dd72aa5baacc3d4759d7477de649f3fef55c2e1e..b40b78b8e7c025d79d855bd3cf697541f35e1817 100644
--- a/chrome/browser/download/download_prefs.cc
+++ b/chrome/browser/download/download_prefs.cc
@@ -153,8 +153,13 @@ bool DownloadPrefs::IsAutoOpenUsed() const {
   return !auto_open_.empty();
 }
 
-bool DownloadPrefs::IsAutoOpenEnabledForExtension(
-    const base::FilePath::StringType& extension) const {
+bool DownloadPrefs::IsAutoOpenEnabledBasedOnExtension(
+    const base::FilePath& path) const {
+  base::FilePath::StringType extension = path.Extension();
+  if (extension.empty())
+    return false;
+  DCHECK(extension[0] == base::FilePath::kExtensionSeparator);
+  extension.erase(0, 1);
   return auto_open_.find(extension) != auto_open_.end();
 }