ASP.NET MVC 3: DataAnnotations.FileExtensionsAttribute not working

Since System.ComponentModel.DataAnnotations.FileExtensionsAttribute is sealed. I use a wrapper for MVC 4.

public class HttpPostedFileExtensionsAttribute : DataTypeAttribute, IClientValidatable
{
    private readonly FileExtensionsAttribute _innerAttribute =
        new FileExtensionsAttribute();

    /// <summary>
    ///     Initializes a new instance of the <see cref="HttpPostedFileExtensionsAttribute" /> class.
    /// </summary>
    public HttpPostedFileExtensionsAttribute()
        : base(DataType.Upload)
    {
        ErrorMessage = _innerAttribute.ErrorMessage;
    }

    /// <summary>
    ///     Gets or sets the file name extensions.
    /// </summary>
    /// <returns>
    ///     The file name extensions, or the default file extensions (".png", ".jpg", ".jpeg", and ".gif") if the property is not set.
    /// </returns>
    public string Extensions
    {
        get { return _innerAttribute.Extensions; }
        set { _innerAttribute.Extensions = value; }
    }

    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata,
        ControllerContext context)
    {
        var rule = new ModelClientValidationRule
        {
            ValidationType = "extension",
            ErrorMessage = FormatErrorMessage(metadata.GetDisplayName())
        };
        rule.ValidationParameters["extension"] = _innerAttribute.Extensions;
        yield return rule;
    }

    /// <summary>
    ///     Applies formatting to an error message, based on the data field where the error occurred.
    /// </summary>
    /// <returns>
    ///     The formatted error message.
    /// </returns>
    /// <param name="name">The name of the field that caused the validation failure.</param>
    public override string FormatErrorMessage(string name)
    {
        return _innerAttribute.FormatErrorMessage(name);
    }

    /// <summary>
    ///     Checks that the specified file name extension or extensions is valid.
    /// </summary>
    /// <returns>
    ///     true if the file name extension is valid; otherwise, false.
    /// </returns>
    /// <param name="value">A comma delimited list of valid file extensions.</param>
    public override bool IsValid(object value)
    {
        var file = value as HttpPostedFileBase;
        if (file != null)
        {
            return _innerAttribute.IsValid(file.FileName);
        }

        return _innerAttribute.IsValid(value);
    }
}