Try one more time to lock down content sniffing for

net/base/mime_util.cc

 // Copyright (c) 2006-2009 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 <string>
 
 #include "net/base/mime_util.h"
 #include "net/base/platform_mime_util.h"
 
 #include "base/hash_tables.h"
@@ skipping 59 matching lines @@
   { "text/css", "css" },
   { "text/xml", "xml" },
   { "image/gif", "gif" },
   { "image/jpeg", "jpeg,jpg" },
   { "image/png", "png" },
   { "video/mp4", "mp4,m4v" },
   { "audio/x-m4a", "m4a" },
   { "audio/mp3", "mp3" },
   { "video/ogg", "ogv,ogm" },
   { "audio/ogg", "ogg,oga" },
-  { "application/xhtml+xml", "xhtml,xht" }
+  { "application/xhtml+xml", "xhtml,xht" },
+  { "application/x-chrome-extension", "crx" }
 };
 
 static const MimeInfo secondary_mappings[] = {
   { "application/octet-stream", "exe,com,bin" },
   { "application/gzip", "gz" },
   { "application/pdf", "pdf" },
   { "application/postscript", "ps,eps,ai" },
   { "application/x-javascript", "js" },
   { "image/bmp", "bmp" },
   { "image/x-icon", "ico" },
@@ skipping 345 matching lines @@
 bool AreSupportedMediaCodecs(const std::vector<std::string>& codecs) {
   return GetMimeUtil()->AreSupportedMediaCodecs(codecs);
 }
 
 void ParseCodecString(const std::string& codecs,
                       std::vector<std::string>* codecs_out) {
   GetMimeUtil()->ParseCodecString(codecs, codecs_out);
 }
 
 }  // namespace net