open png file as ascii-bit(binary)

gistfile1.txt

$ irb-ruby-1.9.2-p290
ruby-1.9.2-p290 :001 > RUBY_VERSION
 => "1.9.2"
ruby-1.9.2-p290 :002 > File.read('test.png').encoding
 => #<Encoding:UTF-8> 
ruby-1.9.2-p290 :003 >  image = File.open('test.png', 'r:ascii-8bit'){|f| f.read}
<snip>
ruby-1.9.2-p290 :004 > image.encoding                                                   
 => #<Encoding:ASCII-8BIT>