The format for the numeric prefix. This is passed to
sprintf(). The default format is "%0Nd" where N
= floor(log10(n)) + 1 and n is the number of files, which means the
prefix may be padded with zeros. For example, if there are 150 files to be
renamed, the format will be "%03d" and the prefixes will be
001, 002, ..., 150.