Call #content_type on MimeType object

This will ensure that the content type is a String.

Fixes thoughtbot#805

lib/paperclip/io_adapters/file_adapter.rb

@@ -63,7 +63,7 @@ def copy_to_tempfile(src)
     end
 
     def best_content_type_option(types)
-      types.reject {|type| type.content_type.match(/\/x-/) }.first
+      types.reject {|type| type.content_type.match(/\/x-/) }.first.content_type
     end
 
     def type_from_file_command

test/file_adapter_test.rb

@@ -23,6 +23,10 @@ class FileAdapterTest < Test::Unit::TestCase
         assert_equal "image/png", @subject.content_type
       end
 
+      should "return content type as a string" do
+        assert_kind_of String, @subject.content_type
+      end
+
       should "get the file's size" do
         assert_equal 4456, @subject.size
       end
@@ -41,6 +45,20 @@ class FileAdapterTest < Test::Unit::TestCase
         assert expected.length > 0
         assert_equal expected, @subject.read
       end
+
+      context "file with multiple possible content type" do
+        setup do
+          MIME::Types.stubs(:type_for).returns([MIME::Type.new('image/x-png'), MIME::Type.new('image/png')])
+        end
+
+        should "prefer officially registered mime type" do
+          assert_equal "image/png", @subject.content_type
+        end
+
+        should "return content type as a string" do
+          assert_kind_of String, @subject.content_type
+        end
+      end
     end
 
     context "empty file" do