Use custom Smart String formatter without ()

Creating a custom formatter | Localization | 1.2.1 (unity3d.com)
Is there a way to use {0:byte} instead of {0:byte()}, just like those built-in formatters?

i think this example can help to you:

using UnityEngine;

public static class ByteExtensions
{
    public static string FormatAsByte(this byte value)
    {
        return value.ToString("byte");
    }
}

Actually I meant the usage in smart string, like “The file size is {0:byte()}”, is there a way to call the formatter without parenthesis “The file size is {0:byte}”