diff --git a/lib/webrat/core/mime.rb b/lib/webrat/core/mime.rb
index 9c55b22..ccff3ec 100644
--- a/lib/webrat/core/mime.rb
+++ b/lib/webrat/core/mime.rb
@@ -1,29 +1,20 @@
+require "rack/mime"
+
 module Webrat #:nodoc:
   module MIME #:nodoc:
+    MIME_TYPES = Rack::Mime::MIME_TYPES.dup.merge(
+      ".multipart_form"   => "multipart/form-data",
+      ".url_encoded_form" => "application/x-www-form-urlencoded"
+    ).freeze
 
-    def self.mime_type(string_or_symbol) #:nodoc:
-      if string_or_symbol.is_a?(String)
-        string_or_symbol
-      else
-        case string_or_symbol
-        when :text              then "text/plain"
-        when :html              then "text/html"
-        when :js                then "text/javascript"
-        when :css               then "text/css"
-        when :ics               then "text/calendar"
-        when :csv               then "text/csv"
-        when :xml               then "application/xml"
-        when :rss               then "application/rss+xml"
-        when :atom              then "application/atom+xml"
-        when :yaml              then "application/x-yaml"
-        when :multipart_form    then "multipart/form-data"
-        when :url_encoded_form  then "application/x-www-form-urlencoded"
-        when :json              then "application/json"
-        else
-          raise ArgumentError.new("Invalid Mime type: #{string_or_symbol.inspect}")
-        end
-      end
+    def mime_type(type)
+      return type if type.nil? || type.to_s.include?("/")
+      type = ".#{type}" unless type.to_s[0] == ?.
+      MIME_TYPES.fetch(type) { |type|
+        raise ArgumentError.new("Invalid Mime type: #{type}")
+      }
     end
 
+    module_function :mime_type
   end
 end
diff --git a/spec/private/core/session_spec.rb b/spec/private/core/session_spec.rb
index 39f4a1c..c5c0ae9 100644
--- a/spec/private/core/session_spec.rb
+++ b/spec/private/core/session_spec.rb
@@ -62,6 +62,11 @@ describe Webrat::Session do
     it "should raise an error if a symbol Mime type is passed that does not exist" do
       lambda { webrat_session.http_accept(:oogabooga) }.should raise_error(ArgumentError)
     end
+
+    it "should recognize a couple of webrat-specific formats" do
+      webrat_session.http_accept(:multipart_form).should == "multipart/form-data"
+      webrat_session.http_accept(:url_encoded_form).should == "application/x-www-form-urlencoded"
+    end
   end
 
   describe "#request_page" do