Umbraco RteCustomValueConverter to output data-rel attributes on images

RteCustomValueConverter.cs

namespace MyProject.ValueConverters
{
    using System.Linq;
    using System.Web;

    using HtmlAgilityPack;

    using Umbraco.Core.Models.PublishedContent;
    using Umbraco.Core.PropertyEditors;
    using Umbraco.Core.PropertyEditors.ValueConverters;
    using Umbraco.Web.PropertyEditors.ValueConverters;

    [PropertyValueType(typeof(IHtmlString))]
    [PropertyValueCache(PropertyCacheValue.All, PropertyCacheLevel.Request)]
    public class RteCustomValueConverter : TinyMceValueConverter
    {
        public override object ConvertDataToSource(PublishedPropertyType propertyType, object source, bool preview)
        {
            if (source == null)
            {
                return null;
            }

            var coreConversion = new RteMacroRenderingValueConverter().ConvertDataToSource(
                propertyType,
                source,
                preview);

            var doc = new HtmlDocument();
            doc.LoadHtml(coreConversion.ToString());

            if (!doc.ParseErrors.Any() && doc.DocumentNode != null)
            {
                // Find all images with rel attribute
                var imgNodes = doc.DocumentNode.SelectNodes("//img[@rel]");
                
                if (imgNodes != null)
                {
                    var modified = false;

                    foreach (var img in imgNodes)
                    {
                        var firstOrDefault = img.Attributes.FirstOrDefault(x => x.Name == "rel");
                        if (firstOrDefault != null)
                        {
                            var rel = firstOrDefault.Value;

                            // Check that the rel attribute is a integer before removing
                            int nodeId;
                            if (int.TryParse(rel, out nodeId))
                            {
                                img.Attributes.Remove("rel");
                                img.Attributes.Add("data-rel", rel);
                                modified = true;
                            }
                        }
                    }

                    if (modified)
                    {
                        return doc.DocumentNode.OuterHtml;
                    }
                }
            }

            return coreConversion;
        }
    }
}

UmbracoEvents.cs

namespace MyProject.Events
{
    using Umbraco.Core;
    using Umbraco.Core.PropertyEditors;
    using Umbraco.Web.PropertyEditors.ValueConverters;

    public class UmbracoEvents : ApplicationEventHandler
    {
        protected override void ApplicationStarting(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
            PropertyValueConvertersResolver.Current.RemoveType<RteMacroRenderingValueConverter>();
        }
    }
}