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

net/http/http_content_disposition.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/http/http_content_disposition.h"
 
 #include "base/logging.h"
 #include "base/string_util.h"
 #include "net/base/net_util.h"
 #include "net/http/http_util.h"
 
 namespace net {
 
 HttpContentDisposition::HttpContentDisposition(
-    const std::string& header, const std::string& referrer_charset)
+    const std::string& header, const std::string& default_charset)
   : type_(INLINE) {
-  Parse(header, referrer_charset);
+  Parse(header, default_charset);
 }
 
 HttpContentDisposition::~HttpContentDisposition() {
 }
 
 std::string::const_iterator HttpContentDisposition::ConsumeDispositionType(
     std::string::const_iterator begin, std::string::const_iterator end) {
   DCHECK(type_ == INLINE);
   std::string::const_iterator delimiter = std::find(begin, end, ';');
 
@@ skipping 26 matching lines @@
 //  disposition-parm    = filename-parm | disp-ext-parm
 //
 //  filename-parm       = "filename" "=" value
 //                      | "filename*" "=" ext-value
 //
 //  disp-ext-parm       = token "=" value
 //                      | ext-token "=" ext-value
 //  ext-token           = <the characters in token, followed by "*">
 //
 void HttpContentDisposition::Parse(const std::string& header,
-                                   const std::string& referrer_charset) {
+                                   const std::string& default_charset) {
   DCHECK(type_ == INLINE);
   DCHECK(filename_.empty());
 
   std::string::const_iterator pos = header.begin();
   std::string::const_iterator end = header.end();
   pos = ConsumeDispositionType(pos, end);
 
   std::string filename;
   std::string ext_filename;
 
   HttpUtil::NameValuePairsIterator iter(pos, end, ';');
   while (iter.GetNext()) {
     if (filename.empty() && LowerCaseEqualsASCII(iter.name_begin(),
                                                  iter.name_end(),
                                                  "filename")) {
-      DecodeFilenameValue(iter.value(), referrer_charset, &filename);
+      DecodeFilenameValue(iter.value(), default_charset, &filename);
     } else if (filename.empty() && LowerCaseEqualsASCII(iter.name_begin(),
                                                         iter.name_end(),
                                                         "name")) {
-      DecodeFilenameValue(iter.value(), referrer_charset, &filename);
+      DecodeFilenameValue(iter.value(), default_charset, &filename);
     } else if (ext_filename.empty() && LowerCaseEqualsASCII(iter.name_begin(),
                                                             iter.name_end(),
                                                             "filename*")) {
       DecodeExtValue(iter.raw_value(), &ext_filename);
     }
   }
 
   filename_ = ext_filename.empty() ? filename : ext_filename;
 }
 
 }  // namespace net