Allow to register a mime type with mime_type

lib/sinatra/base.rb

@@ -730,11 +730,12 @@ module Sinatra
         end
       end
 
-      # Look up a media type by file extension in Rack's mime registry.
-      def mime_type(type)
+      # Lookup or register a mime type in Rack's mime registry.
+      def mime_type(type, value=nil)
         return type if type.nil? || type.to_s.include?('/')
         type = ".#{type}" unless type.to_s[0] == ?.
-        Rack::Mime.mime_type(type, nil)
+        return Rack::Mime.mime_type(type, nil) unless value
+        Rack::Mime::MIME_TYPES[type] = value
       end
 
       # Define a before filter. Filters are run before all requests

test/helpers_test.rb

@@ -230,6 +230,19 @@ class HelpersTest < Test::Unit::TestCase
     end
   end
 
+  test 'Base.mime_type registers mime type' do
+    mock_app {
+      mime_type :foo, 'application/foo'
+
+      get '/' do
+        "foo is #{mime_type(:foo)}"
+      end
+    }
+
+    get '/'
+    assert_equal 'foo is application/foo', body
+  end
+
   describe 'content_type' do
     it 'sets the Content-Type header' do
       mock_app {