UrlExtension for rendering a Content URL to a physical file, adding the modification date as a URL parameter. Explanation at https://stefanolsen.com/posts/cache-busting-with-aspnet-mvc/

UrlExtensions.cs

public static class UrlExtensions
{
    private const string FileDateTicksCacheKeyFormat = "FileDateTicks_{0}";

    private static long GetFileDateTicks(this UrlHelper urlHelper, string filename)
    {
        var context = urlHelper.RequestContext.HttpContext;
        string cacheKey = string.Format(FileDateTicksCacheKeyFormat, filename);

        // Check if we already cached the ticks in the cache.
        if (context.Cache[cacheKey] != null)
        {
            return (long)context.Cache[cacheKey];
        }

        var physicalPath = context.Server.MapPath(filename);
        var fileInfo = new FileInfo(physicalPath);
        var dependency = new CacheDependency(physicalPath);

        // If file exists, read number of ticks from last write date. Or fall back to 0.
        long ticks = fileInfo.Exists ? fileInfo.LastWriteTime.Ticks : 0;

        // Add the number of ticks to cache for 12 hours.
        // The cache dependency will remove the entry if file is changed or deleted.
        context.Cache.Add(cacheKey, ticks, dependency,
            DateTime.Now.AddHours(12), TimeSpan.Zero,
            CacheItemPriority.Normal, null);

        return ticks;
    }

    public static string ContentVersioned(this UrlHelper urlHelper, string contentPath)
    {
        string url = urlHelper.Content(contentPath);

        long fileTicks = GetFileDateTicks(urlHelper, url);

        return $"{url}?v={fileTicks}";
    }
}