Don't #reprocess! if there is no file.

lib/paperclip/attachment.rb

@@ -159,14 +159,15 @@ module Paperclip
     # again.
     def reprocess!
       new_original = Tempfile.new("paperclip-reprocess")
-      old_original = to_file(:original)
-      new_original.write( old_original.read )
-      new_original.rewind
+      if old_original = to_file(:original)
+        new_original.write( old_original.read )
+        new_original.rewind
 
-      @queued_for_write = { :original => new_original }
-      post_process
+        @queued_for_write = { :original => new_original }
+        post_process
 
-      old_original.close if old_original.respond_to?(:close)
+        old_original.close if old_original.respond_to?(:close)
+      end
     end
 
     private