Error file upload

gistfile1.txt

From params:

#<ActionDispatch::Http::UploadedFile:0x00000003aa01d8 @original_filename="58.png",
 @content_type="image/png",
 @headers="Content-Disposition: form-data; name=\"bug[screen]\"; filename=\"58.png\"\r\nContent-Type: image/png\r\n",
 @tempfile=#<File:/tmp/RackMultipart20120514-17845-2755c4>>

--------------------------------------------------------
In the model: 

  def savefile(file_up)
    name = file_up.original_filename
    File.open(Rails.root.join('public', 'uploads', name), 'w') do |file|
      file.write(file_up.read)
    end    
  end

In the conroller: 
@bug.savefile params[:bug][:screen]
-----------------------------------------------------------
Error:
Encoding::UndefinedConversionError in BugsController#create
"\x89" from ASCII-8BIT to UTF-8

Trace:
app/models/bug.rb:17:in `write'
app/models/bug.rb:17:in `puts'
app/models/bug.rb:17:in `block in savefile'
app/models/bug.rb:16:in `open'
app/models/bug.rb:16:in `savefile'
app/controllers/bugs_controller.rb:24:in `create'