validating file upload asp.net core mvc

 public class AllowedExtensionsAttribute:ValidationAttribute
{
    private readonly string[] _Extensions;
    public AllowedExtensionsAttribute(string[] Extensions)
    {
        _Extensions = Extensions;
    }

    protected override ValidationResult IsValid(
    object value, ValidationContext validationContext)
    {
        var file = value as IFormFile;
        var extension = Path.GetExtension(file.FileName);
        if (!(file == null))
        {
            if (!_Extensions.Contains(extension.ToLower()))
            {
                return new ValidationResult(GetErrorMessage());
            }
        }

        return ValidationResult.Success;
    }

    public string GetErrorMessage()
    {
        return $"This photo extension is not allowed!";
    }
}public class UserViewModel
{
        [Required(ErrorMessage = "Please select a file.")]
        [DataType(DataType.Upload)]
        [MaxFileSize(5* 1024 * 1024)]
        [AllowedExtensions(new string[] { ".jpg", ".png" })]
        public IFormFile Photo { get; set; }
 }