def format(file)
      type = File.extname(file).sub('.','')
      type = DEFAULT_FORMAT if type.empty?
      type
    end
def format(file)
      type = File.extname(file).sub('.','')
      type = DEFAULT_FORMAT if type.empty?
      type
    end
The format of the file based on the extension.
 If the file has no extension then the value of
 `DEFAULT_FORMAT` is returned.

 @return [String]