Saw an issue with a nil content_type sometimes in Safari. Fix by doing to_s.

lib/paperclip/attachment.rb

@@ -77,7 +77,7 @@ module Paperclip
       logger.info("[paperclip] Writing attributes for #{name}")
       @queued_for_write[:original]   = uploaded_file.to_tempfile
       instance_write(:file_name,       uploaded_file.original_filename.strip.gsub(/[^\w\d\.\-]+/, '_'))
-      instance_write(:content_type,    uploaded_file.content_type.strip)
+      instance_write(:content_type,    uploaded_file.content_type.to_s.strip)
       instance_write(:file_size,       uploaded_file.size.to_i)
       instance_write(:updated_at,      Time.now)