From 43ab0158fbcafb13fa25d3a06dd53f7b7a92f196 Mon Sep 17 00:00:00 2001
From: Trevor Turk <trevorturk@gmail.com>
Date: Thu, 28 Feb 2013 14:02:56 -0600
Subject: Document introduction of the ActionController::UnknownFormat
 exception

---
 guides/source/upgrading_ruby_on_rails.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index 52f5232efc..d378d51c2a 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -101,6 +101,8 @@ Also check your environment settings for `config.action_dispatch.best_standards_
 
 * In Rails 4.0, precompiling assets no longer automatically copies non-JS/CSS assets from `vendor/assets` and `lib/assets`. Rails application and engine developers should put these assets in `app/assets` or configure `config.assets.precompile`.
 
+* In Rails 4.0, a rescuable exception `ActionController::UnknownFormat` is raised when Rails doesn't know what to do with the request format, rather than responding with a head :not_acceptable (406).
+
 ### Active Support
 
 Rails 4.0 removes the `j` alias for `ERB::Util#json_escape` since `j` is already used for `ActionView::Helpers::JavaScriptHelper#escape_javascript`.
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