[RequiredCollection]

public ICollection<IFormFile> Attachments { get; set; }

public class RequiredCollectionAttribute : ValidationAttribute

{

    protected const string DefaultErrorMessageFormatString = "You must provide at least one.";

    public RequiredCollectionAttribute() : base(DefaultErrorMessageFormatString) { }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)

    {

        var collection = (ICollection) value;

        return collection == null || collection.Count >

            ? ValidationResult.Success

            : new ValidationResult(ErrorMessageString);

    }

}

[HttpPost]

public async Task<IActionResult> Method(MethodViewModel viewModel)

{

    if (!ModelState.IsValid)

        return View(viewModel);

    ...

}