@using (Ajax.BeginForm("Upload", "Attachment", FormMethod.Post, 
new AjaxOptions
                            {
                                HttpMethod = "POST",
                            },
                            new
                            {
                                encType = "multipart/form-data",
                                id = "attach-form"
                            }))
{
    @Html.AntiForgeryToken()

    @Html.TextBoxFor(m => m.FirstName })
    
    <input type="file" name="Files" data-buttonText="انتخاب تصویر">
   
    <button type="submit">ارسال</button>
}

        var formData = new FormData();

        $('form').submit(function() {
            
            var action = $(this).attr('action');
            var formData = new FormData($(this).get(0));

            $.ajax({
                type: "POST",
                dataType: "json",
                url: action,
                data: formData,
                processData: false,
                contentType: false,
                success: function(data) {
                    //...
                }
                success: function(data) {
                     //...
                }
            });

            return false;
        });

public class MyModel
    {
        public string FirstName{ get; set; }

        public IEnumerable<HttpPostedFileBase> Files { get; set; }
    }  

[AjaxOnly]
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Upload(MyModel model)
{
     if (!ModelState.IsValid)
         return //....

     if(model.Files != null)
         foreach (var file in model.Files)
             if (file  != null && file.ContentLength > 0)
             {
                 // ....
             }
}

var formData = new FormData();
var splitedKeywords = $('#keywords').val().split(',');
formData.append('Keywords', JSON.stringify(splitedKeywords));

"\"کلمه اول\",\"کلمه دوم\""

var formData = new FormData();
var splitedKeywords = $('#keywords').val().split(',');
for (var counter = 0; counter < splitedKeywords.length; counter++) {
    formData.append('Keywords', JSON.stringify(splitedKeywords[counter]));
}

$.post( "manage/test", { name: "John", time: "2pm" })
  .done(function( data ) {
    alert( "Data Loaded: " + data );
  });

var formData = new FormData();
            formData.append('Organizations', $('#Organizations').val());
            formData.append('OrgNameInReports', $('#OrgNameInReports').val());
            formData.append('CenterTypes', $('#CenterTypes').val());
            formData.append('Users', $('#Users').val());
            formData.append('Comments', $('#Comments').val());
            formData.append('UserTypes', $('#UserTypes').val());

            jQuery.each($('#profile-img')[0].files, function (i, file) {
                formData.append('picture-' + i, file);
            });
            $.ajax({                
                url: 'SettingPages/OrgManagmentService.asmx/SaveNewOrgManager',
                type: 'POST',
                data: formData,
                 dataType: 'json',
                contentType: false,
                processData: false,

                beforeSend: function () {
                    Preloader();
                },
                complete: function (data) {
                    
                    RemovePreloader();
                },
                success: function (data, status) { 
                    data = JSON.stringify(data);
                    console.log(data);
                   

                },
                error: function (data, status,e) {
                    alert(status);
                }
            });