File Extension Validation Attribute C# .NET

ImageFileAttribute.cs

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))
            {
                return false;
            }

            return true;
        }
        return false;
    }
}