ASP.NET Core Multiple File Upload Validation

FileAttribute.cs

public class FileAttribute : ValidationAttribute
{
  public string[] FileTypes { get; set; }
  protected override ValidationResult IsValid(object value, ValidationContext validationContext)
  {
    if (value is IFormFile)
    {
      return ValidateSingleFile((IFormFile)value);
    }
    else if (value is IEnumerable<IFormFile>)
    {
      return ValidateMultipleFiles(value);
    }
    else
    {
      return new ValidationResult("The input type is not a file.");
    }
  }

  private ValidationResult ValidateSingleFile(IFormFile file)
  {
    if (!isFileAllowedType(file))
    {
      return DisallowedFileTypeError(file);
    }

    return ValidationResult.Success;
  }

  private ValidationResult ValidateMultipleFiles(object value)
  {
    IEnumerable<IFormFile> files = (IEnumerable<IFormFile>)value;

    foreach (var file in files)
    {
        if (!isFileAllowedType(file))
        {
            return DisallowedFileTypeError(file);
        }
    }

    return ValidationResult.Success;
  }

  private bool isFileAllowedType(IFormFile file)
  {
    string fileType = file.ContentType.ToLower();
    return FileTypes.Length == 0 || FileTypes.Contains(fileType);
  }

  private ValidationResult DisallowedFileTypeError(IFormFile file)
  {
    string fileName = file.FileName;
    return new ValidationResult($"{ fileName } is not an allowed file type.");
  }
}