Well spotted.

After removing the patch, the file size is reported properly ONLY after processing. If the files aren't processed - 0 is reported.

This might work for JRuby (iostream.rb):

if defined? Tempfile
  class Tempfile
    def size
      if self.path
        File.size(self.path)
      else
        0
      end
    end
  end
end

Since paperclip's Tempfile monkeypatch is meant to fix a bug on Windows, it should just be patched on Windows:

if defined? Tempfile and /mswin|win32|mingw|bccwin|cygwin/ =~ RUBY_PLATFORM
  class Tempfile
    def size
      if @tmpfile
        @tmpfile.fsync
        @tmpfile.flush
        @tmpfile.stat.size
      else
        0
      end
    end
  end
end