根据byte计算文件大小

- (id)transformedValue:(NSString *)value{    double convertedValue = [value doubleValue];    int multiplyFactor = 0;    NSArray *tokens = [NSArray arrayWithObjects:@"bytes",@"KB",@"MB",@"GB",@"TB",@"PB", @"EB", @"ZB", @"YB",nil];    while (convertedValue > 1024) {        convertedValue /= 1024;        multiplyFactor++;    }    return [NSString stringWithFormat:@"%4.1f%@",convertedValue, [tokens objectAtIndex:multiplyFactor]];}