LinkItemAllowedTypesAttribute.cs

using EPiServer;
using EPiServer.DataAnnotations;
using EPiServer.SpecializedProperties;
using EPiServer.Web.Routing;
using System;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Web.ModelBinding;

namespace LinkItemAllowedTypes.Business.Attributes
{
    [AttributeUsage(AttributeTargets.Property)]
    public class LinkItemAllowedTypesAttribute : ValidationAttribute, IMetadataAware
    {
        public Type[] AllowedTypes { get; set; }

        public LinkItemAllowedTypesAttribute(Type[] allowedTypes)
        {
            AllowedTypes = allowedTypes;
        }

        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            if (value == null)
            {
                return null;
            }

            if (!(value is LinkItemCollection))
            {
                return ValidationResult.Success;
            }

            if (((LinkItemCollection)value).Count <= 0)
            {
                return ValidationResult.Success;
            }

            foreach (var linkItem in (LinkItemCollection)value)
            {
                var linkItemValidated = false;

                var content = UrlResolver.Current.Route(new UrlBuilder(linkItem.Href));

                if (content == null)
                {
                    continue;
                }

                var originalType = content.GetOriginalType();

                foreach (var allowedPageType in AllowedTypes)
                {
                    if (!allowedPageType.IsAssignableFrom(originalType))
                    {
                        continue;
                    }

                    linkItemValidated = true;
                    break;

                }

                if (linkItemValidated)
                {
                    continue;
                }

                var validationErrorResult = new ValidationResult
                (ErrorMessage = string.Format(
                    App_GlobalResources.ValidationMessagesResources.LinkItemTypeNotAllowedErrorMessage,
                    GetDisplayName(originalType), validationContext.DisplayName));

                return !string.IsNullOrEmpty(ErrorMessage) ?
                    new ValidationResult(ErrorMessage) : validationErrorResult;
            }

            return ValidationResult.Success;
        }

        public void OnMetadataCreated(ModelMetadata metadata)
        {
            //throw new NotImplementedException();
        }

        public override bool RequiresValidationContext
        {
            get
            {
                return true;
            }
        }

        private static string GetDisplayName(Type type)
        {
            var contentTypeAttribute = type.GetCustomAttributes(typeof(ContentTypeAttribute), true)
                .FirstOrDefault() as ContentTypeAttribute;

            var displayName = contentTypeAttribute?.DisplayName;

            return !string.IsNullOrEmpty(displayName) ? displayName : "";
        }
    }
}

This version of the LinkItemAllowedTypesAttribute supports inherited types.
The implementation depends on the existence of App_GlobalResources.ValidationMessagesResources.LinkItemTypeNotAllowedErrorMessage