Yea, I had found that one already. I couldn't get it to work. However, I just did some picking-apart on it and I found a simplified version of a key line actually worked.

I took this line:

dynamic umbracoFile = JsonConvert.DeserializeObject(image.GetValue<string>(Constants.Conventions.Media.File));

And changed it to:

string url = image.GetValue<string>(Constants.Conventions.Media.File);

Thank you your encouragement.

For anyone else out there, here is my working code. This will iterate through the file tree, starting with a root folder of your choice (by ID), and create an unordered list of folders and files, with links to the files, that is collapsible.

Controller:

    using Microsoft.AspNetCore.Mvc;
    using Umbraco.Cms.Core;
    using Umbraco.Cms.Core.Models;
    using Umbraco.Cms.Core.Services;

    namespace Prod.Controllers    
    {
        [Route("[controller]/[action]")]
        public class MyController : Controller
        {
            private readonly IMediaService mediaService;

            public MyController(IMediaService mediaService)
            {
                this.mediaService = mediaService;               
            }           


            [HttpPost]
            public ActionResult GetTree()
            {
                LoadCurrentTree();

                return PartialView("My/View");
            }

            public void LoadCurrentTree()
            {
                var output = "";
                var folder = mediaService.GetById(1250);

                GetBranch(ref output, folder, true);

                ViewBag.CurrentTree = output;
            }

            public void GetBranch(ref string output, IMedia media, bool top)
            {
                output += "<ul";
                if (!top)
                    output += " class='nested'";
                output += ">";

                var children = mediaService.GetPagedChildren(media.Id, 0, 100, out var total);

                foreach(var child in children)
                {                    
                    output += "<li>";
                    if(child.ContentType.Alias == "Folder")
                    {

                        output += "<span class='caret'>" + child.Name + "</span>";
                        GetBranch(ref output, child, false);
                    } else
                    {                        
                        string url = child.GetValue<string>(Constants.Conventions.Media.File);       

                        output += "<a href='" + url + "' target='_blank'>" + child.Name + "</a>";
                    }
                    output += "</li>";
                }
                output += "</ul>";
            }
       }
}

Scripts:

<script type="text/javascript">
    $(function () {        
        $(".caret").click(function(){   
            $(this).next(".nested").toggleClass("active");
            $(this).toggleClass("caret-down");
        });
    });
</script>

Style:

<style>
    /* Remove default bullets */
    ul {
      list-style-type: none;
    }

    /* Style the caret/arrow */
    .caret {
      cursor: pointer;
      user-select: none; /* Prevent text selection */
    }

    /* Create the caret/arrow with a unicode, and style it */
    .caret::before {
      content: "\25B6";
      color: black;
      display: inline-block;
      margin-right: 6px;
    }

    /* Rotate the caret/arrow icon when clicked on (using JavaScript) */
    .caret-down::before {
      transform: rotate(90deg);
    }

    /* Hide the nested list */
    .nested {
      display: none;
    }

    /* Show the nested list when the user clicks on the caret/arrow (with JavaScript) */
    .active {
      display: block;
      padding-bottom: 20px;
    }
</style>

View:

@Html.Raw(ViewBag.CurrentTree)