Dodecahedrane

public class ExceptionName : Exception
{
    public ExceptionName() { }

    public ExceptionName(string message) : base(message) { }

    public ExceptionName(string message, Exception innerException) : base(message, innerException) { }
}

Dodecahedrane

public class ImageFile : ValidationAttribute
{
    private readonly string[] _allowedExtensions = { ".png", ".jpeg", ".gif" };

    public override bool IsValid(object value)
    {
        if (value is IFormFile file)
        {
            var fileExtension = System.IO.Path.GetExtension(file.FileName).ToLower();
            if (!_allowedExtensions.Contains(fileExtension))