diff --git a/atom/browser/ui/webui/pdf_viewer_ui.cc b/atom/browser/ui/webui/pdf_viewer_ui.cc
index 8221c0cf97bb..5b1afc411f05 100644
--- a/atom/browser/ui/webui/pdf_viewer_ui.cc
+++ b/atom/browser/ui/webui/pdf_viewer_ui.cc
@@ -78,8 +78,8 @@ class BundledDataSource : public content::URLDataSource {
   }
 
   std::string GetMimeType(const std::string& path) const override {
-    auto file = base::FilePath(PathWithoutParams(path));
-    base::FilePath::StringType ext = file.Extension();
+    base::FilePath::StringType ext =
+        base::FilePath::FromUTF8Unsafe(PathWithoutParams(path)).Extension();
     std::string mime_type;
     if (!ext.empty() &&
         net::GetWellKnownMimeTypeFromExtension(ext.substr(1), &mime_type))