2 files changed, 24 insertions, 2 deletions
diff --git a/actionpack/lib/action_dispatch/http/mime_type.rb b/actionpack/lib/action_dispatch/http/mime_type.rb
index ee1913dbf9..fe39c220a5 100644
--- a/actionpack/lib/action_dispatch/http/mime_type.rb
+++ b/actionpack/lib/action_dispatch/http/mime_type.rb
@@ -29,6 +29,11 @@ module Mime
     Type.lookup_by_extension(type.to_s)
   end
 
+  def self.fetch(type)
+    return type if type.is_a?(Type)
+    EXTENSION_LOOKUP.fetch(type.to_s) { |k| yield k }
+  end
+
   # Encapsulates the notion of a mime type. Can be used at render time, for example, with:
   #
   #   class PostsController < ActionController::Base
@@ -53,6 +58,8 @@ module Mime
     cattr_reader :browser_generated_types
     attr_reader :symbol
 
+    @register_callbacks = []
+
     # A simple helper class used in parsing the accept header
     class AcceptItem #:nodoc:
       attr_accessor :order, :name, :q
@@ -84,6 +91,10 @@ module Mime
       TRAILING_STAR_REGEXP = /(text|application)\/\*/
       PARAMETER_SEPARATOR_REGEXP = /;\s*\w+="?\w+"?/
 
+      def register_callback(&block)
+        @register_callbacks << block
+      end
+
       def lookup(string)
         LOOKUP[string]
       end
@@ -101,10 +112,15 @@ module Mime
       def register(string, symbol, mime_type_synonyms = [], extension_synonyms = [], skip_lookup = false)
         Mime.const_set(symbol.upcase, Type.new(string, symbol, mime_type_synonyms))
 
-        SET << Mime.const_get(symbol.upcase)
+        new_mime = Mime.const_get(symbol.upcase)
+        SET << new_mime
 
         ([string] + mime_type_synonyms).each { |str| LOOKUP[str] = SET.last } unless skip_lookup
         ([symbol] + extension_synonyms).each { |ext| EXTENSION_LOOKUP[ext.to_s] = SET.last }
+
+        @register_callbacks.each do |callback|
+          callback.call(new_mime)
+        end
       end
 
       def parse(accept_header)
diff --git a/actionpack/lib/action_dispatch/testing/test_request.rb b/actionpack/lib/action_dispatch/testing/test_request.rb
index a86b510719..639ae6f398 100644
--- a/actionpack/lib/action_dispatch/testing/test_request.rb
+++ b/actionpack/lib/action_dispatch/testing/test_request.rb
@@ -12,7 +12,7 @@ module ActionDispatch
 
     def initialize(env = {})
       env = Rails.application.env_config.merge(env) if defined?(Rails.application) && Rails.application
-      super(DEFAULT_ENV.merge(env))
+      super(default_env.merge(env))
 
       self.host        = 'test.host'
       self.remote_addr = '0.0.0.0'
@@ -69,5 +69,11 @@ module ActionDispatch
     def cookies
       @cookies ||= {}.with_indifferent_access
     end
+
+    private
+
+    def default_env
+      DEFAULT_ENV
+    end
   end
 end