Try matching uploaded file type before defaulting to first official one

and memoize type_from_file_command

lib/paperclip/content_type_detector.rb

@@ -44,12 +44,13 @@ module Paperclip
     end
 
     def best_type_match
+      types_matching_file = possible_types.select {|type| type.content_type == type_from_file_command}
       official_types = possible_types.reject {|type| type.content_type.match(/\/x-/) }
-      (official_types.first || possible_types.first).content_type
+      (types_matching_file.first || official_types.first || possible_types.first).content_type
     end
 
     def type_from_file_command
-      FileCommandContentTypeDetector.new(@filename).detect
+      @type_from_file_command ||= FileCommandContentTypeDetector.new(@filename).detect
     end
 
   end