Add download_file_types.proto with ascii->binary conversion, as a resource.

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 in download_file_types.asciipb

[asanka, 2016/04/06 02:12:31]

Embedded underscores could be interpreted as _emphasis_.

[Nathan Parker, 2016/04/11 20:12:07]

yea, hrm.  I thought intra-word was disabled.  I'll see how it looks rendered,
and maybe adjust it. I suspect most people will read it in .txt form.

+is both baked into Chrome and pushable to Chrome between releases.
+
+Rendered version of this file: https://chromium.googlesource.com/chromium/src/+/master/chrome/browser/resources/safe_browsing/README.md
+
+
+# Procedure for adding a new type
+  * Edit `download_file_types.asciipb`. Update `histograms.xml`
+  * Get it reviewed, submit.
+  * Push via component update (PROCEDURE TBD)
+
+# Guidelines for a DownloadFileType entry:
+See `download_file_types.proto` for all fields.
+  * `extension` value must be unique within the config. It should be
+    lowercase ASCII and not contain a dot.  If there _is_ a duplicate,
+    last one will win.
+  * `uma_value` value must be unique and match one in the
+    SBClientDownloadExtensions enum in histograms.xml
+  * `is_archive`: True if this filetype is a container for other files.
+  * `platform_settings`: (repeated) Must have one entry with platform=ALL, and
+     optionally additional entries with overrides for one or more
+     platforms.  platform=ALL acts as a default for any platforms that
+     don't have an override.  There shouldn't be two settings with the
+     same `platform`, but if there are, last one wins.  Keep them sorted
+     by platform.
+  * `platform_settings.danger_level`: Required.
+    * `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.
+  * `platform_settings.auto_open_hint`: Required.
+    * `ALLOW_AUTO_OPEN`: File type can be opened automatically if the user
+      selected that option from the download tray on a previous download
+      of this type.
+    * `DISALLOW_AUTO_OPEN`:  Never let the file automatically open.
+      Files that should be disallowed from auto-opening include those that
+      execute arbitrary or harmful code with user privileges, or change
+      configuration of the system to cause harmful behavior immediately
+      or at some time in the future. We *do* allow auto-open for files
+      that upon opening sufficiently warn the user about the fact that it
+      was downloaded from the internet and can do damage.  (TODO(asanka):
+      Clarify this sentence: We don't warn for files that...
+       "do nothing particularly dangerous, despite the act
+        of downloading itself being dangerous (E.g. .local and .manifest
+        files)).
+  * `platform_settings.ping_setting`:  Required.  This controls what sort
+     of ping is sent to Safe Browsing and if a verdict is checked before
+     the user can access the file.
+    * `SAMPLE_WITH_LIGHT_PING`: Don't send a full Safe Browsing ping, but
+       send a no-PII "light-ping" for a random sample of SBER users.
+       This should be the default for unknown types.  The verdict won't be
+       used.
+    * `NO_PING`:  Don’t send any pings.  This file is whitelisted. All
+      NOT_DANGEROUS files should normally use this.
+    * `FULL_PING`: Send full pings and use the verdict. All dangerous
+      file should use this.
+
+# Guidelines for the top level DownloadFileTypeConfig entry:
+  * `version_id`: Must be increased (+1) every time the file is checked in.
+     Will be logged to UMA.
+
+  * `light_ping_probability`: For what fraction of extended-reporting users'
+    downloads with unknown extensions (or ping_setting=TREAT_AS_UNKNOWN)
+    should we send light-pings? [0..1]
+
+  * `file_type`: The big list of all known file types. Keep them
+     sorted by extension.
+
+  * `default_file_type`: Settings used if a file is not in the above list.
+    `extension` is ignored, but other settings are used.  The ping_setting
+    should be TREAT_AS_UNKNOWN for all platforms.
+