Neutralize dangerous subresource files during Save Page.

chrome/browser/resources/safe_browsing/README.md

 # Behavior of Download File Types in Chrome
 
 This describes how to adjust file-type download behavior in
 Chrome including interactions with Safe Browsing. The metadata described
 here, and stored in `download_file_types.asciipb`, will be both baked into
 Chrome released and pushable to Chrome between releases (via
 `FileTypePolicies` class).  http://crbug.com/596555
 
 Rendered version of this file: https://chromium.googlesource.com/chromium/src/+/master/chrome/browser/resources/safe_browsing/README.md
 
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        1. If there's an entry matching the built platform,
          that will be preferred. Otherwise,
 
        2. If there's a "PLATFORM_ANY" (i.e. `platform` is not set),
        that will be used. Otherwise,
 
        3. The `default_file_type`'s settings will be filled in.
 
   * `platform_settings.danger_level`: (required)
+    Note that this policy affects how individual file downloads are handled as
+    well as how subresources are handled for *"Save As ..."* downloads of
+    complete web pages.
+
+    For *"Save As ..."* downloads, if any subresource ends up with a file type
+    that is considered `DANGEROUS` or `ALLOW_ON_USER_GESTURE`, then the filename
+    will be changed to end in `.download`. This is done to prevent the file from
+    being opened accidentally.
+
     * `NOT_DANGEROUS`: Safe to download and open, even if the download
        was accidental.
     * `DANGEROUS`: Always warn the user that this file may harm their
       computer. We let them continue or discard the file. If Safe
       Browsing returns a SAFE verdict, we still warn the user.
     * `ALLOW_ON_USER_GESTURE`: Warn the user normally but skip the warning
       if there was a user gesture or the user visited this site before
       midnight last night (i.e. is a repeat visit). If Safe Browsing
       returns a SAFE verdict for this file, it won't show a warning.
 
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     users' downloads with unknown extensions (or
     ping_setting=SAMPLED_PING) should we send light-pings? [0.0 .. 1.0]
 
   * `file_types`: The big list of all known file types. Keep them
      sorted by extension.
 
   * `default_file_type`: Settings used if a downloaded file is not in
     the above list. `extension` is ignored, but other settings are used.
     The ping_setting should be SAMPLED_PING for all platforms.