Add a missing require statement on top of upfile.rb

Closes #508, Closes #511

lib/paperclip/upfile.rb

+require 'mime/types'
+
 module Paperclip
 module Paperclip
   # The Upfile module is a convenience module for adding uploaded-file-type methods
   # The Upfile module is a convenience module for adding uploaded-file-type methods
   # to the +File+ class. Useful for testing.
   # to the +File+ class. Useful for testing.