Give the filter setup more context so it can figure out whether it's download...

net/base/load_flags.h

 // Copyright (c) 2006-2008 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.
 
 #ifndef NET_BASE_LOAD_FLAGS_H__
 #define NET_BASE_LOAD_FLAGS_H__
 
 namespace net {
 
 // These flags provide metadata about the type of the load request.  They are
@@ skipping 20 matching lines @@
   LOAD_DISABLE_CACHE = 1 << 4,
 
   // This is a navigation that will not be intercepted by any registered
   // URLRequest::Interceptors.
   LOAD_DISABLE_INTERCEPT = 1 << 5,
 
   // If present, upload progress messages should be provided to initiator.
   LOAD_ENABLE_UPLOAD_PROGRESS = 1 << 6,
 
   // If present, try to download the resource to a standalone file.
+  // This hint tells the http cache to provide a native file handle
+  // of the cached file to the renderer process.
   LOAD_ENABLE_DOWNLOAD_FILE = 1 << 7,
 
   // If present, ignores certificate mismatches with the domain name.
   // (The default behavior is to trigger an OnSSLCertificateError callback.)
   LOAD_IGNORE_CERT_COMMON_NAME_INVALID = 1 << 8,
 
   // If present, ignores certificate expiration dates
   // (The default behavior is to trigger an OnSSLCertificateError callback).
   LOAD_IGNORE_CERT_DATE_INVALID = 1 << 9,
 
@@ skipping 13 matching lines @@
   // cookies or updating existing ones.
   LOAD_DO_NOT_SAVE_COOKIES = 1 << 13,
 
   // An SDCH dictionary was advertised, and an SDCH encoded response is
   // possible.
   LOAD_SDCH_DICTIONARY_ADVERTISED = 1 << 14,
 
   // Do not resolve proxies. This override is used when downloading PAC files
   // to avoid having a circular dependency.
   LOAD_BYPASS_PROXY = 1 << 15,
+
+  // Indicate this request is for a download, as opposed to viewing.
+  LOAD_IS_DOWNLOAD = 1 << 16,
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_LOAD_FLAGS_H__