referrer_charset is a lie. It's really the user's default_charset.

net/http/http_content_disposition.h

Index: net/http/http_content_disposition.h
===================================================================
--- net/http/http_content_disposition.h	(revision 119865)
+++ net/http/http_content_disposition.h	(working copy)
@@ -21,7 +21,7 @@
   };
 
   HttpContentDisposition(const std::string& header,
-                         const std::string& referrer_charset);
+                         const std::string& default_charset);
   ~HttpContentDisposition();
 
   bool is_attachment() const { return type() == ATTACHMENT; }
@@ -30,7 +30,7 @@
   const std::string& filename() const { return filename_; }
 
  private:
-  void Parse(const std::string& header, const std::string& referrer_charset);
+  void Parse(const std::string& header, const std::string& default_charset);
   std::string::const_iterator ConsumeDispositionType(
       std::string::const_iterator begin, std::string::const_iterator end);