gistfile1.rbx

## FORM

- form_for @user, :html => { :multipart => true } do |f|

  ...

  %p
    = f.file_field :avatar

## MODEL

  has_attached_file :avatar, :styles => { :small => "150x150>", :large => "200x200>" }#,
  validates_attachment_size :avatar, :less_than => 2.megabyte
  validates_attachment_content_type :avatar, :content_type => ["images/jpeg", "image/png"]