Paperclip Validation

has_attached_file :avatar,
    :styles => { :medium => "200x200>", :thumb => "100x100>" }

validates_attachment_content_type :avatar,
    :content_type => ['image/jpg', 'image/jpeg', 'image/pjpeg', 'image/gif', 'image/png', 'image/x-png'],
    :message => "only image files are allowed"

validates_attachment_size :avatar,
    :less_than => 1.megabyte, #another option is :greater_than
    :message => "max size is 1M"

限制到k的话可以用300.kilobytes

限制尺寸
validates_attachment_presence :mp3