Automatically close file while checking mime type

[fixes #2016]

lib/paperclip/content_type_detector.rb

@@ -67,8 +67,9 @@ module Paperclip
     end
 
     def type_from_mime_magic
-      @type_from_mime_magic ||=
-        MimeMagic.by_magic(File.open(@filepath)).try(:type)
+      @type_from_mime_magic ||= File.open(@filepath) do |file|
+        MimeMagic.by_magic(file).try(:type)
+      end
     end
 
     def type_from_file_command