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This function instruct PIL to use function 'func' as a new URL to filename
conversion routine. This function will be called during validation
of any parameter of type file with access checking on ('fr', 'fw', 'fn' or
PIL's default function is the one which speaks CFITSIO's language.
It is valid to call this function with func set to NULL. In this case
any URL will be treated verbatim as a filename during filename