def _guess_file_format(filename):
    """Guess the file format from the filename.

	Parameters
	----------
	filename : string
		The filename of the file.

	Returns
	-------
	string
		The file type. This is one of ["asdf", "fits"].
	"""
    if filename.endswith('asdf'):
        return 'asdf'
    elif filename.endswith('fits') or filename.endswith('fits.gz'):
        return 'fits'
    else:
        return None
