Вы должны предоставить перегрузку, которая опускает необязательный параметр:
/// <summary>
/// Sets data associated with the instance using the default media type.
/// </summary>
/// <param name="key">The key defining the data.</param>
/// <param name="value">The data.</param>
public void SetData(object key, object value)
{
SetData(key, value, null);
}
/// <summary>
/// Sets data associated with the instance using the specified media type.
/// </summary>
/// <param name="key">The key defining the data.</param>
/// <param name="value">The data.</param>
/// <param name="mime">The media type of the data.</param>
public void SetData(object key, object value, string mime)
{
...
}
В качестве альтернативы, вы можете объявить параметр необязательным:
/// <summary>
/// Sets data associated with the instance.
/// </summary>
/// <param name="key">The key defining the data.</param>
/// <param name="value">The data.</param>
/// <param name="mime">The media type of the data.</param>
public void SetData(object key, object value, string mime = null)
{
...
}