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]];
}