def delete_file_conditionally
      unless self.class.where(key: key).where(['id != ?', id]).exists?
        storage.delete key
      end
    end
def delete_file_conditionally
      unless self.class.where(key: key).where(['id != ?', id]).exists?
        storage.delete key
      end
    end
Deletes the file on disk if and only if no other instances of EchoUpload::File
 reference it.