jellyfin/MediaBrowser.Controller/Providers/LocalTrailerProvider.cs

using System.Collections.Generic;
using System.ComponentModel.Composition;
using System.IO;
using System.Threading.Tasks;
using MediaBrowser.Controller.Events;
using MediaBrowser.Model.Entities;

namespace MediaBrowser.Controller.Providers
{
    [Export(typeof(BaseMetadataProvider))]
    public class LocalTrailerProvider : BaseMetadataProvider
    {
        public async override Task Fetch(BaseItem item, ItemResolveEventArgs args)
        {
            var trailerPath = args.GetFolderByName("trailers");

            if (trailerPath.HasValue)
            {
                string[] allFiles = Directory.GetFileSystemEntries(trailerPath.Value.Key, "*", SearchOption.TopDirectoryOnly);

                List<Video> localTrailers = new List<Video>();

                foreach (string file in allFiles)
                {
                    BaseItem child = await Kernel.Instance.ItemController.GetItem(null, file);

                    Video video = child as Video;

                    if (video != null)
                    {
                        localTrailers.Add(video);
                    }
                }

                item.LocalTrailers = localTrailers;
            }
        }
    }
}