https://www.progress.com/documentation/sitefinity-cms/extend-the-content-list-widget

ExtendedContentListEntity.cs

using Progress.Sitefinity.AspNetCore;
using Progress.Sitefinity.AspNetCore.Widgets.Models.ContentList;
using Progress.Sitefinity.Renderer.Designers.Attributes;
using Progress.Sitefinity.Renderer.Entities.Content;

namespace SandboxWebApp.Entities
{
    /// <summary>
    /// Extended entity class for the ContentBlock view component.
    /// </summary>
    public class ExtendedContentListEntity : ContentListEntity
    {
        /// <inheritdoc />
        [ContentSection(Constants.ContentSectionTitles.SelectContentToDisplay, 0)]
        [Content(
            Type = $"{KnownContentTypes.ListItems}, {KnownContentTypes.News}",
            TypeBlacklist = "Telerik.Sitefinity.DynamicTypes.Model.PressReleases.Pressrelease"
        )]
        public override MixedContentContext SelectedItems
        {
            get
            {
                return base.SelectedItems;
            }
            set
            {
                base.SelectedItems = value;
            }
        }
    }
}