CustomMedia.cs

using Sitecore.Resources.Media;
using Sitecore.Data.Items;
using System.Web;

namespace MyProject.Foundation.Media
{
    public class CustomMedia : Media
    {
        private const string ForceDownloadFieldName = "Force Download";

        public override MediaStream GetStream()
        {
            MediaItem mediaItem = this.MediaData.MediaItem;

            if (IsForceDownloadEnabled(mediaItem))
            {
                this.SetAttachmentDisposition(HttpContext.Current.Response, mediaItem.Name, mediaItem.Extension);
            }

            return base.GetStream();
        }

        private bool IsForceDownloadEnabled(Item item)
        {
            var field = item.Fields[ForceDownloadFieldName];

            return field != null && (Sitecore.Data.Fields.CheckboxField)field;
        }

        private void SetAttachmentDisposition(HttpResponse response, string name, string extension)
        {
            string filename = $"{name}.{extension}";

            response.Headers["Content-Disposition"] = $"attachment; filename=\"{filename}\"";
        }
    }
}