Paperclip Resize to fit a rectangular box

I have a rectangular image for example 30x800 pixels

How I can scale it with paperclip to preserve the aspect ratio to a 100x100 pixel image with borders filling the empty area ?

an example : http://www.imagemagick.org/Usage/thumbnails/pad_extent.gif

  has_attached_file :image, :styles => { :thumb => "100x100>" }, 
    :convert_options => {:thumb => "-gravity center -extent 100x100"}

Or with not white background

  has_attached_file :image, :styles => { :thumb => "100x100>" }, 
    :convert_options => {:thumb => "-background red -gravity center -extent 100x100"}