diff --git a/actionpack/lib/action_dispatch/http/mime_type.rb b/actionpack/lib/action_dispatch/http/mime_type.rb
index ba6cf809ca..001bac977d 100644
--- a/actionpack/lib/action_dispatch/http/mime_type.rb
+++ b/actionpack/lib/action_dispatch/http/mime_type.rb
@@ -2,6 +2,7 @@ require 'set'
 require 'singleton'
 require 'active_support/core_ext/module/attribute_accessors'
 require 'active_support/core_ext/string/starts_ends_with'
+require 'active_support/deprecation'
 
 module Mime
   class Mimes < Array
@@ -35,6 +36,40 @@ module Mime
       return type if type.is_a?(Type)
       EXTENSION_LOOKUP.fetch(type.to_s) { |k| yield k }
     end
+
+    def const_missing(sym)
+      if Mime::Type.registered?(sym)
+        ActiveSupport::Deprecation.warn <<-eow
+Accessing mime types via constants is deprecated.  Please change:
+
+  `Mime::#{sym}`
+
+to:
+
+  `Mime::Type[:#{sym}]`
+        eow
+        Mime::Type[sym]
+      else
+        super
+      end
+    end
+
+    def const_defined?(sym, inherit = true)
+      if Mime::Type.registered?(sym)
+        ActiveSupport::Deprecation.warn <<-eow
+Accessing mime types via constants is deprecated.  Please change:
+
+  `Mime::#{sym}`
+
+to:
+
+  `Mime::Type[:#{sym}]`
+        eow
+        true
+      else
+        super
+      end
+    end
   end
 
   # Encapsulates the notion of a mime type. Can be used at render time, for example, with:
@@ -66,7 +101,7 @@ module Mime
       def initialize(index, name, q = nil)
         @index = index
         @name = name
-        q ||= 0.0 if @name == Mime::ALL.to_s # default wildcard match to end of list
+        q ||= 0.0 if @name == Mime::Type[:ALL].to_s # default wildcard match to end of list
         @q = ((q || 1.0).to_f * 100).to_i
       end
 
@@ -120,7 +155,7 @@ module Mime
         end
 
         def app_xml_idx
-          @app_xml_idx ||= index(Mime::XML.to_s)
+          @app_xml_idx ||= index(Mime::Type[:XML].to_s)
         end
 
         def text_xml
@@ -137,6 +172,8 @@ module Mime
         end
     end
 
+    TYPES = {}
+
     class << self
       TRAILING_STAR_REGEXP = /(text|application)\/\*/
       PARAMETER_SEPARATOR_REGEXP = /;\s*\w+="?\w+"?/
@@ -145,6 +182,18 @@ module Mime
         @register_callbacks << block
       end
 
+      def registered?(symbol)
+        TYPES.key? symbol
+      end
+
+      def [](symbol)
+        TYPES[symbol]
+      end
+
+      def add_type(symbol, type)
+        TYPES[symbol] = type
+      end
+
       def lookup(string)
         LOOKUP[string]
       end
@@ -161,7 +210,7 @@ module Mime
 
       def register(string, symbol, mime_type_synonyms = [], extension_synonyms = [], skip_lookup = false)
         new_mime = Type.new(string, symbol, mime_type_synonyms)
-        Mime.const_set(symbol.upcase, new_mime)
+        add_type symbol.upcase, new_mime
 
         SET << new_mime
 
@@ -216,8 +265,7 @@ module Mime
       #   Mime::Type.unregister(:mobile)
       def unregister(symbol)
         symbol = symbol.upcase
-        mime = Mime.const_get(symbol)
-        Mime.instance_eval { remove_const(symbol) }
+        mime = TYPES.delete symbol
 
         SET.delete_if { |v| v.eql?(mime) }
         LOOKUP.delete_if { |_,v| v.eql?(mime) }
diff --git a/actionpack/lib/action_dispatch/http/mime_types.rb b/actionpack/lib/action_dispatch/http/mime_types.rb
index 01a10c693b..d349cf2807 100644
--- a/actionpack/lib/action_dispatch/http/mime_types.rb
+++ b/actionpack/lib/action_dispatch/http/mime_types.rb
@@ -33,4 +33,4 @@ Mime::Type.register "application/pdf", :pdf, [], %w(pdf)
 Mime::Type.register "application/zip", :zip, [], %w(zip)
 
 # Create Mime::ALL but do not add it to the SET.
-Mime::ALL = Mime::Type.new("*/*", :all, [])
+Mime::Type.add_type :ALL, Mime::Type.new("*/*", :all, [])
diff --git a/actionpack/lib/action_dispatch/http/parameters.rb b/actionpack/lib/action_dispatch/http/parameters.rb
index 27fadb708e..e3c4392760 100644
--- a/actionpack/lib/action_dispatch/http/parameters.rb
+++ b/actionpack/lib/action_dispatch/http/parameters.rb
@@ -4,7 +4,7 @@ module ActionDispatch
       PARAMETERS_KEY = 'action_dispatch.request.path_parameters'
 
       DEFAULT_PARSERS = {
-        Mime::JSON => lambda { |raw_post|
+        Mime::Type[:JSON] => lambda { |raw_post|
           data = ActiveSupport::JSON.decode(raw_post)
           data.is_a?(Hash) ? data : {:_json => data}
         }
diff --git a/actionpack/test/dispatch/mime_type_test.rb b/actionpack/test/dispatch/mime_type_test.rb
index 3017a9c2d6..5a2d9a1d3f 100644
--- a/actionpack/test/dispatch/mime_type_test.rb
+++ b/actionpack/test/dispatch/mime_type_test.rb
@@ -13,76 +13,75 @@ class MimeTypeTest < ActiveSupport::TestCase
   test "unregister" do
     begin
       Mime::Type.register("text/x-mobile", :mobile)
-      assert defined?(Mime::MOBILE)
-      assert_equal Mime::MOBILE, Mime::LOOKUP['text/x-mobile']
-      assert_equal Mime::MOBILE, Mime::EXTENSION_LOOKUP['mobile']
+      assert Mime.const_defined?(:MOBILE)
+      assert_equal Mime::Type[:MOBILE], Mime::LOOKUP['text/x-mobile']
+      assert_equal Mime::Type[:MOBILE], Mime::EXTENSION_LOOKUP['mobile']
 
       Mime::Type.unregister(:mobile)
-      assert !defined?(Mime::MOBILE), "Mime::MOBILE should not be defined"
+      assert !Mime.const_defined?(:MOBILE), "Mime::MOBILE should not be defined"
       assert !Mime::LOOKUP.has_key?('text/x-mobile'), "Mime::LOOKUP should not have key ['text/x-mobile]"
       assert !Mime::EXTENSION_LOOKUP.has_key?('mobile'), "Mime::EXTENSION_LOOKUP should not have key ['mobile]"
     ensure
-      Mime.module_eval { remove_const :MOBILE if const_defined?(:MOBILE) }
       Mime::LOOKUP.reject!{|key,_| key == 'text/x-mobile'}
     end
   end
 
   test "parse text with trailing star at the beginning" do
     accept = "text/*, text/html, application/json, multipart/form-data"
-    expect = [Mime::HTML, Mime::TEXT, Mime::JS, Mime::CSS, Mime::ICS, Mime::CSV, Mime::VCF, Mime::XML, Mime::YAML, Mime::JSON, Mime::MULTIPART_FORM]
+    expect = [Mime::Type[:HTML], Mime::Type[:TEXT], Mime::Type[:JS], Mime::Type[:CSS], Mime::Type[:ICS], Mime::Type[:CSV], Mime::Type[:VCF], Mime::Type[:XML], Mime::Type[:YAML], Mime::Type[:JSON], Mime::Type[:MULTIPART_FORM]]
     parsed = Mime::Type.parse(accept)
     assert_equal expect, parsed
   end
 
   test "parse text with trailing star in the end" do
     accept = "text/html, application/json, multipart/form-data, text/*"
-    expect = [Mime::HTML, Mime::JSON, Mime::MULTIPART_FORM, Mime::TEXT, Mime::JS, Mime::CSS, Mime::ICS, Mime::CSV, Mime::VCF, Mime::XML, Mime::YAML]
+    expect = [Mime::Type[:HTML], Mime::Type[:JSON], Mime::Type[:MULTIPART_FORM], Mime::Type[:TEXT], Mime::Type[:JS], Mime::Type[:CSS], Mime::Type[:ICS], Mime::Type[:CSV], Mime::Type[:VCF], Mime::Type[:XML], Mime::Type[:YAML]]
     parsed = Mime::Type.parse(accept)
     assert_equal expect, parsed
   end
 
   test "parse text with trailing star" do
     accept = "text/*"
-    expect = [Mime::HTML, Mime::TEXT, Mime::JS, Mime::CSS, Mime::ICS, Mime::CSV, Mime::VCF, Mime::XML, Mime::YAML, Mime::JSON]
+    expect = [Mime::Type[:HTML], Mime::Type[:TEXT], Mime::Type[:JS], Mime::Type[:CSS], Mime::Type[:ICS], Mime::Type[:CSV], Mime::Type[:VCF], Mime::Type[:XML], Mime::Type[:YAML], Mime::Type[:JSON]]
     parsed = Mime::Type.parse(accept)
     assert_equal expect, parsed
   end
 
   test "parse application with trailing star" do
     accept = "application/*"
-    expect = [Mime::HTML, Mime::JS, Mime::XML, Mime::RSS, Mime::ATOM, Mime::YAML, Mime::URL_ENCODED_FORM, Mime::JSON, Mime::PDF, Mime::ZIP]
+    expect = [Mime::Type[:HTML], Mime::Type[:JS], Mime::Type[:XML], Mime::Type[:RSS], Mime::Type[:ATOM], Mime::Type[:YAML], Mime::Type[:URL_ENCODED_FORM], Mime::Type[:JSON], Mime::Type[:PDF], Mime::Type[:ZIP]]
     parsed = Mime::Type.parse(accept)
     assert_equal expect, parsed
   end
 
   test "parse without q" do
     accept = "text/xml,application/xhtml+xml,text/yaml,application/xml,text/html,image/png,text/plain,application/pdf,*/*"
-    expect = [Mime::HTML, Mime::XML, Mime::YAML, Mime::PNG, Mime::TEXT, Mime::PDF, Mime::ALL]
+    expect = [Mime::Type[:HTML], Mime::Type[:XML], Mime::Type[:YAML], Mime::Type[:PNG], Mime::Type[:TEXT], Mime::Type[:PDF], Mime::Type[:ALL]]
     assert_equal expect, Mime::Type.parse(accept)
   end
 
   test "parse with q" do
     accept = "text/xml,application/xhtml+xml,text/yaml; q=0.3,application/xml,text/html; q=0.8,image/png,text/plain; q=0.5,application/pdf,*/*; q=0.2"
-    expect = [Mime::HTML, Mime::XML, Mime::PNG, Mime::PDF, Mime::TEXT, Mime::YAML, Mime::ALL]
+    expect = [Mime::Type[:HTML], Mime::Type[:XML], Mime::Type[:PNG], Mime::Type[:PDF], Mime::Type[:TEXT], Mime::Type[:YAML], Mime::Type[:ALL]]
     assert_equal expect, Mime::Type.parse(accept)
   end
 
   test "parse single media range with q" do
     accept = "text/html;q=0.9"
-    expect = [Mime::HTML]
+    expect = [Mime::Type[:HTML]]
     assert_equal expect, Mime::Type.parse(accept)
   end
 
   test "parse arbitrary media type parameters" do
     accept = 'multipart/form-data; boundary="simple boundary"'
-    expect = [Mime::MULTIPART_FORM]
+    expect = [Mime::Type[:MULTIPART_FORM]]
     assert_equal expect, Mime::Type.parse(accept)
   end
 
   # Accept header send with user HTTP_USER_AGENT: Sunrise/0.42j (Windows XP)
   test "parse broken acceptlines" do
     accept = "text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/*,,*/*;q=0.5"
-    expect = [Mime::HTML, Mime::XML, "image/*", Mime::TEXT, Mime::ALL]
+    expect = [Mime::Type[:HTML], Mime::Type[:XML], "image/*", Mime::Type[:TEXT], Mime::Type[:ALL]]
     assert_equal expect, Mime::Type.parse(accept).collect(&:to_s)
   end
 
@@ -90,16 +89,14 @@ class MimeTypeTest < ActiveSupport::TestCase
   #  (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322; InfoPath.1)
   test "parse other broken acceptlines" do
     accept = "image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/x-shockwave-flash, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword,  , pronto/1.00.00, sslvpn/1.00.00.00, */*"
-    expect = ['image/gif', 'image/x-xbitmap', 'image/jpeg','image/pjpeg', 'application/x-shockwave-flash', 'application/vnd.ms-excel', 'application/vnd.ms-powerpoint', 'application/msword', 'pronto/1.00.00', 'sslvpn/1.00.00.00', Mime::ALL]
+    expect = ['image/gif', 'image/x-xbitmap', 'image/jpeg','image/pjpeg', 'application/x-shockwave-flash', 'application/vnd.ms-excel', 'application/vnd.ms-powerpoint', 'application/msword', 'pronto/1.00.00', 'sslvpn/1.00.00.00', Mime::Type[:ALL]]
     assert_equal expect, Mime::Type.parse(accept).collect(&:to_s)
   end
 
   test "custom type" do
     begin
       Mime::Type.register("image/foo", :foo)
-      assert_nothing_raised do
-        assert_equal Mime::FOO, Mime::SET.last
-      end
+      assert_equal Mime::Type[:FOO], Mime::SET.last
     ensure
       Mime::Type.unregister(:FOO)
     end
@@ -109,7 +106,7 @@ class MimeTypeTest < ActiveSupport::TestCase
     begin
       Mime::Type.register "text/foobar", :foobar, ["text/foo", "text/bar"]
       %w[text/foobar text/foo text/bar].each do |type|
-        assert_equal Mime::FOOBAR, type
+        assert_equal Mime::Type[:FOOBAR], type
       end
     ensure
       Mime::Type.unregister(:FOOBAR)
@@ -124,7 +121,7 @@ class MimeTypeTest < ActiveSupport::TestCase
       end
 
       Mime::Type.register("text/foo", :foo)
-      assert_equal [Mime::FOO], registered_mimes
+      assert_equal [Mime::Type[:FOO]], registered_mimes
     ensure
       Mime::Type.unregister(:FOO)
     end
@@ -134,7 +131,7 @@ class MimeTypeTest < ActiveSupport::TestCase
     begin
       Mime::Type.register "text/foobar", :foobar, [], [:foo, "bar"]
       %w[foobar foo bar].each do |extension|
-        assert_equal Mime::FOOBAR, Mime::EXTENSION_LOOKUP[extension]
+        assert_equal Mime::Type[:FOOBAR], Mime::EXTENSION_LOOKUP[extension]
       end
     ensure
       Mime::Type.unregister(:FOOBAR)
@@ -144,15 +141,15 @@ class MimeTypeTest < ActiveSupport::TestCase
   test "register alias" do
     begin
       Mime::Type.register_alias "application/xhtml+xml", :foobar
-      assert_equal Mime::HTML, Mime::EXTENSION_LOOKUP['foobar']
+      assert_equal Mime::Type[:HTML], Mime::EXTENSION_LOOKUP['foobar']
     ensure
       Mime::Type.unregister(:FOOBAR)
     end
   end
 
   test "type should be equal to symbol" do
-    assert_equal Mime::HTML, 'application/xhtml+xml'
-    assert_equal Mime::HTML, :html
+    assert_equal Mime::Type[:HTML], 'application/xhtml+xml'
+    assert_equal Mime::Type[:HTML], :html
   end
 
   test "type convenience methods" do
@@ -162,9 +159,8 @@ class MimeTypeTest < ActiveSupport::TestCase
     # Remove custom Mime::Type instances set in other tests, like Mime::GIF and Mime::IPHONE
     types.delete_if { |type| !Mime.const_defined?(type.upcase) }
 
-
     types.each do |type|
-      mime = Mime.const_get(type.upcase)
+      mime = Mime::Type[type.upcase]
       assert mime.respond_to?("#{type}?"), "#{mime.inspect} does not respond to #{type}?"
       assert mime.send("#{type}?"), "#{mime.inspect} is not #{type}?"
       invalid_types = types - [type]
@@ -174,8 +170,14 @@ class MimeTypeTest < ActiveSupport::TestCase
   end
 
   test "mime all is html" do
-    assert Mime::ALL.all?,  "Mime::ALL is not all?"
-    assert Mime::ALL.html?, "Mime::ALL is not html?"
+    assert Mime::Type[:ALL].all?,  "Mime::ALL is not all?"
+    assert Mime::Type[:ALL].html?, "Mime::ALL is not html?"
+  end
+
+  test "deprecated lookup" do
+    assert_deprecated do
+      assert Mime::ALL.all?,  "Mime::ALL is not all?"
+    end
   end
 
   test "verifiable mime types" do
@@ -186,18 +188,18 @@ class MimeTypeTest < ActiveSupport::TestCase
   end
 
   test "references gives preference to symbols before strings" do
-    assert_equal :html, Mime::HTML.ref
+    assert_equal :html, Mime::Type[:HTML].ref
     another = Mime::Type.lookup("foo/bar")
     assert_nil another.to_sym
     assert_equal "foo/bar", another.ref
   end
 
   test "regexp matcher" do
-    assert Mime::JS =~ "text/javascript"
-    assert Mime::JS =~ "application/javascript"
-    assert Mime::JS !~ "text/html"
-    assert !(Mime::JS !~ "text/javascript")
-    assert !(Mime::JS !~ "application/javascript")
-    assert Mime::HTML =~ 'application/xhtml+xml'
+    assert Mime::Type[:JS] =~ "text/javascript"
+    assert Mime::Type[:JS] =~ "application/javascript"
+    assert Mime::Type[:JS] !~ "text/html"
+    assert !(Mime::Type[:JS] !~ "text/javascript")
+    assert !(Mime::Type[:JS] !~ "application/javascript")
+    assert Mime::Type[:HTML] =~ 'application/xhtml+xml'
   end
 end