Upload Files to Sitefinity Asynchronously with Kendo UI Upload widget - https://sitefinitydevelopment.com/blog/uploading-files-to-sitefinity-asynchronously-using-kendo-ui-upload.html

FileAsyncUploaderController.cs

using System.Web.Mvc;
using Telerik.Sitefinity.Mvc;

namespace SitefinityWebApp.Mvc.Controllers
{
    [ControllerToolboxItem(Name = "FileAsyncUploader", Title = "File Async Uploader", SectionName = "Custom MVC", CssClass = "sfMvcIcn")]
    public class FileAsyncUploaderController: Controller
    {
        [HttpGet]
        public ActionResult Index()
        {
            return View("Index");
        }

        [HttpPost]
        public ActionResult Upload()
        {
            // save the file to Sitefinity
            // put here just for illustrative purposes
                        
            return new EmptyResult();
        }
    }
}

FilesController.cs

using System;
using System.Net;
using System.Net.Http;
using System.Web.Http;
using Telerik.Sitefinity.Abstractions;
using System.Linq;
using System.Collections.Generic;
using System.Web;
using System.IO;
using Telerik.Sitefinity.Libraries.Model;
using Telerik.Sitefinity.Modules.Libraries;
using Telerik.Sitefinity.Workflow;
using Telerik.Sitefinity.Data;
using System.Text.RegularExpressions;
using Telerik.Sitefinity.Security.Claims;
using Telerik.Sitefinity.GenericContent.Model;

namespace SitefinityWebApp.Mvc.Api
{
    public class FilesController : ApiController
    {
        [HttpPost]
        public HttpResponseMessage Upload()
        {
            var user = ClaimsManager.GetCurrentIdentity();

            if (!user.IsAuthenticated)
            {
                return Request.CreateErrorResponse(HttpStatusCode.Forbidden, "You must be authenticated to upload files");
            }

            var error = string.Empty;

            var files = HttpContext.Current.Request.Files;

            if (files != null)
            {
                var manager = LibrariesManager.GetManager();

                using (var reg = new ElevatedModeRegion(manager))
                {
                    // create a library with the name of the logged in user
                    var libTitle = user.Name;

                    var library = GetOrCreateDocumentsLibrary(libTitle);

                    for (int i = 0; i < files.Count; i++)
                    {
                        var file = files[i];

                        try
                        {
                            // Some browsers send file names with full path. 
                            // This needs to be stripped.
                            var fileName = Path.GetFileName(file.FileName);
                            var ext = Path.GetExtension(file.FileName).ToLower();

                            var docId = Guid.NewGuid();
                            var doc = manager.CreateDocument(docId);

                            var utcNow = DateTime.UtcNow;

                            doc.Parent = library;
                            doc.Title = fileName;
                            doc.DateCreated = utcNow;
                            doc.PublicationDate = utcNow;
                            doc.LastModified = utcNow;

                            var urlName = CreateSitefinityUrlFromString(fileName);
                            doc.UrlName = doc.MediaFileUrlName = urlName;

                            manager.Upload(doc, file.InputStream, ext);

                            manager.RecompileItemUrls(doc);

                            manager.SaveChanges();

                            var bag = new Dictionary<string, string>();
                            bag.Add("ContentType", typeof(Document).FullName);
                            WorkflowManager.MessageWorkflow(docId, typeof(Document), null, "Publish", false, bag);
                        }
                        catch (Exception ex)
                        {
                            Log.Write(ex.ToString());
                            error = "Failed to upload: " + file.FileName + ex.ToString();
                        }
                    }                    
                }
            }

            // Return an empty string to signify success or an error message otherwise
            return Request.CreateResponse(HttpStatusCode.OK, error);
        }

        [HttpPost]
        public HttpResponseMessage Remove([FromBody] RemoveFileModel filesToRemove)
        {
            var user = ClaimsManager.GetCurrentIdentity();

            if (!user.IsAuthenticated)
            {
                return Request.CreateErrorResponse(HttpStatusCode.Forbidden, "You must be authenticated to upload files");
            }

            var manager = LibrariesManager.GetManager();

            using (var region = new ElevatedModeRegion(manager))
            {
                var library = manager.GetDocumentLibraries()
                                     .Where(l => l.Title == user.Name)
                                     .Single();

                var fileNames = filesToRemove.fileNames;

                var docToDelete = library.Documents()
                                         .Where(d => d.Status == ContentLifecycleStatus.Master)
                                         .Where(d => fileNames.Contains(d.Title.ToString()))
                                         .ToList();

                docToDelete.ForEach(d => manager.DeleteDocument(d));
                
                manager.SaveChanges();
            }

            return Request.CreateResponse(HttpStatusCode.OK);
        }

        public Library GetOrCreateDocumentsLibrary(string title)
        {
            var librariesManager = LibrariesManager.GetManager();
            Library library = null;

            using (var region = new ElevatedModeRegion(librariesManager))
            {
                var libs = librariesManager.GetDocumentLibraries().Where(l => l.Title == title);
                if (libs.Count() == 0)
                {
                    library = librariesManager.CreateDocumentLibrary();
                    library.Title = title;
                    library.DateCreated = DateTime.UtcNow;
                    library.LastModified = DateTime.UtcNow;
                    library.UrlName = CreateSitefinityUrlFromString(title);

                    librariesManager.RecompileAndValidateUrls(library);
                    librariesManager.SaveChanges();
                }
                else
                {
                    library = libs.First();
                }
            }
            return library;
        }

        public string CreateSitefinityUrlFromString(string title, bool isTitleNotUnique = false)
        {
            var url = Regex.Replace(title.ToLower(), @"[^\w\-\!\$\'\(\)\=\@\d_]+", "-");

            if (isTitleNotUnique)
            {
                url += Guid.NewGuid().ToString();
            }

            return url;
        }
    }

    public class RemoveFileModel
    {
        // the name of the files to be deleted
        public string[] fileNames { get; set; }
    }
}

global.asax.cs

using System;
using System.Linq;
using System.Web.Hosting;
using System.Web.Http;
using System.Web.Optimization;
using System.Web.Routing;
using Telerik.Sitefinity.Services;

namespace SitefinityWebApp
{
    public class Global : System.Web.HttpApplication
    {
        protected void Application_Start(object sender, EventArgs e)
        {
            SystemManager.ApplicationStart += SystemManager_ApplicationStart;
        }

        private void SystemManager_ApplicationStart(object sender, EventArgs e)
        {
            RegisterRoutes(RouteTable.Routes);
        }

        private void RegisterRoutes(RouteCollection routes)
        {
            routes.MapHttpRoute(
                name: "MyCustomApi",
                routeTemplate: "ajax/{controller}/{action}/{id}",
                defaults: new { id = RouteParameter.Optional }
                );
        }
    }
}

Index.cshtml

@using Telerik.Sitefinity.Frontend.Mvc.Helpers;
@using Telerik.Sitefinity.Modules.Pages;

@Html.Script(ScriptRef.KendoWeb, "bottom")

@Html.StyleSheet("//kendo.cdn.telerik.com/2017.2.504/styles/kendo.common-bootstrap.min.css", "head")
@Html.StyleSheet("//kendo.cdn.telerik.com/2017.2.504/styles/kendo.bootstrap.min.css", "head")

<div class="row">
    <div class="col-md-4">
        <div class="form-group">
            <label>Upload Files</label>
            <input name="files" id="files" type="file" />
        </div>
    </div>
</div>

<script>
    $(function() {

        var saveUrl = '/ajax/files/upload';
        var removeUrl = '/ajax/files/remove';

        $("#files").kendoUpload({
            multiple: true,
            async: {
                saveUrl: saveUrl,
                removeUrl: removeUrl,
                autoUpload: true,
                batch: true
            }
        });

    })
</script>