Default to Paperclip::ContentTypeDetector (#2270)

* Add default `content_type_detector` to `UploadedFileAdapter`

lib/paperclip/io_adapters/uploaded_file_adapter.rb

@@ -24,7 +24,7 @@ module Paperclip
     end
 
     def content_type_detector
-      self.class.content_type_detector
+      self.class.content_type_detector || Paperclip::ContentTypeDetector
     end
 
     def determine_content_type

spec/paperclip/io_adapters/uploaded_file_adapter_spec.rb

@@ -29,7 +29,7 @@ describe Paperclip::UploadedFileAdapter do
       end
 
       it "gets the content type" do
-        assert_equal "image/x-png-by-browser", @subject.content_type
+        assert_equal "image/png", @subject.content_type
       end
 
       it "gets the file's size" do
@@ -98,7 +98,7 @@ describe Paperclip::UploadedFileAdapter do
       end
 
       it "gets the content type" do
-        assert_equal "image/x-png-by-browser", @subject.content_type
+        assert_equal "image/png", @subject.content_type
       end
 
       it "gets the file's size" do