Gist Criado para integração do TinyMCE ASP.NET MVC

Create.cshtml

@model EuVotoAf.Models.ViewModels.PublicaoViewModel

@{
    ViewBag.Title = "Create";
    Layout = "~/Views/Shared/_Layout.cshtml";
}
<script src="~/Scripts/tinymce/tinymce.js"></script>
<h2>Create</h2>


@using (Html.BeginForm("Create", "Publicacao", FormMethod.Post))
{
    @Html.AntiForgeryToken()

    <div class="row">
        <div class="col-md-6">
            <label class="control-label col-md-4">Titulo</label>
            @Html.TextBoxFor(model => model.Titulo, new { @class = "form-control col-md-4", @placeholder = "Digite o Titulo" })
        </div>
    </div>

   <div>
        @Html.TextAreaFor(model => model.Conteudo, new { id = "my_editor", @class = "mceEditor" })
    </div>

    <div class="form-group">
        <div class="col-md-offset-2 col-md-10">
            <input type="submit" value="Create" class="btn btn-default" />
        </div>
    </div>
}

<input name="file" type="file" id="file">

@section Scripts{
    @Scripts.Render("~/bundles/jqueryval")
    <script src="//cdn.tinymce.com/4/tinymce.min.js"></script>
    <script type="text/javascript">
        tinymce.init({
            selector: "textarea",
            theme: "modern",
            width: 700,
            plugins: [
                "advlist autolink lists link charmap print preview hr anchor pagebreak",
                "searchreplace wordcount visualblocks visualchars code fullscreen",
                "insertdatetime media nonbreaking save table contextmenu directionality",
                "emoticons template paste textcolor colorpicker textpattern imagetools"
            ],
            toolbar1: "insertfile undo redo | styleselect | bold italic | alignleft aligncenter alignright alignjustify | bullist numlist outdent indent | link image",
            toolbar2: "print preview media | forecolor backcolor emoticons | ltr rtl",
            image_advtab: true,
            templates: [
                { title: 'Test template 1', content: 'Test 1' },
                { title: 'Test template 2', content: 'Test 2' }
            ],
            file_browser_callback: function (field_name, url, type, win) {
                if (type == 'image') $('#my_form input').click();
            }
        });
    </script>

    <script>
        $('#file').change(function () {
            var formData = new FormData();
            var file = this.files[0];
            console.log(file)
            formData.append('file', file);
            $.ajax({
                url: '@Url.Action("Upload", "Publicacao")',
                type: 'POST',
                data: formData,
                processData: false,
                contentType: false,
                success: function (data) {
                    console.log(data)
                    tinyMCE.get('my_editor').setContent('Digite seu texto');
                    tinymce.activeEditor.execCommand('mceInsertContent', false, data);
                }
            });
        });
    </script>
}

Upload.cs

        public string Upload(HttpPostedFileBase file)
        {
            string path;
            string saveloc = "~/Uploads/Imagens";
            string filename = file.FileName;

            if (file != null && file.ContentLength > 0)
            {
                try
                {
                    path = Path.Combine(HttpContext.Server.MapPath(saveloc), Path.GetFileName(filename));
                    file.SaveAs(path);
                }
                catch (Exception e)
                {
                    return "<script>alert('Failed: " + e + "');</script>";
                }
            }
            else
            {
                return "<script>alert('Failed: Unkown Error. This form only accepts valid images.');</script>";
            }

            return @"<img src=/Uploads/Imagens/" + filename + " >";
        }