Rails - Apr 2006 - image content-type question

If I want to check that someone has uploaded a standard image, would
simply doing

class Image
  def validate
      errors.add(:content_type, "must be an image") unless
content_type and content_type.include?("image")
  end
end

So, basically checking to make sure that the content type of the image
contains "image".  Will that backfire at any point?  I suppose I could
check for "image/jpeg" or "image/gif" or whatever.