send form using ajax in mvc

I have this wiered behavior in my project and I've followed this, this and this links but with no success. Everytime my controller method gets hit while debugging, the model will be always null. Can anyone tell what else methods I can try to pass the model through form.serialize()

Here is my Model:

public class AddNewUser
{
     public string uname { get; set; }
     public string password { get; set; }
     public string Role { get; set; }
     public string fname { get; set; }
     public string lname { get; set; }
     public HttpPostedFileBase profilepic { get; set; }
     public string email { get; set; }
     public string contact { get; set; }
}

Here is my view:

@model AdminViewModel.AddNewUser
@{
    var uRole = TempData["UserRole"] as string;
    TempData.Keep("UserRole");
}

@using (Html.BeginForm("AddUser", "Admin", FormMethod.Post, new { id = "frmAddUser", @class = "form-admin col-lg-8 col-lg-offset-2" }))
{

    <h2 class="form-login-heading mar-pad">Add new user</h2>

     <label style="font-weight:normal" for="#txtuserName">Username *</label>
     @Html.TextBoxFor(m => m.uname, new {type="text",name="uname",tabindex="1",id="txtuserName", style="padding-right:20px", autocomplete="off", spellcheck="false",placeholder="Enter username", @class="form-control ip" })

     <label style="font-weight:normal" for="#txtPassword">Password *</label>

     @Html.PasswordFor(m => m.password, new { name = "password", tabindex = "2", id = "txtPassword", autocomplete = "off", spellcheck = "false", @class = "form-control ip", placeholder = "Enter password", data_tog = "tooltip", title = "Your Password must contain : <br />- at least 1 upper case character, <br /> - at least 1 lower case character, <br /> - at least 1 numerical character, <br /> - at least 1 special character.", data_placement = "top" })

     @if (uRole == "Admin")
     {

             <label style="font-weight:normal;margin-left:15px !important" for="#selectRole">Select Role *</label><br />
             <select id="selectRole" class="selectpicker show-menu-arrow full_wid col-md-12" tabindex="3" data-size="5">
                   <option value="0" selected disabled>Please select a role</option>
                   <option value="1">Admin</option>
                   <option value="2">Manager</option>
             </select>

             @Html.HiddenFor(m => m.Role, new { id="hdnRole", data_selected=""})

             <label style="font-weight:normal" for="#txtFName">First name *</label>
             @Html.TextBoxFor(m => m.fname, new { id="txtFname", style="padding-right:20px !important;", tabindex="4", autocomplete="off", spellcheck="false", placeholder="Enter user first name", @class="form-control ip"})

             <label style="font-weight:normal" for="#txtLName">Last name *</label>
             @Html.TextBoxFor(m => m.lname, new { id = "txtLname", style = "padding-right:20px !important;", tabindex = "5", autocomplete = "off", spellcheck = "false", placeholder = "Enter user last name", @class = "form-control ip"})

       }
       else
       {
             <label style="font-weight:normal" for="#txtFName">First name *</label>
             @Html.TextBoxFor(m => m.fname, new { id = "txtFname", style = "padding-right:20px !important;", tabindex = "3", autocomplete = "off", spellcheck = "false", placeholder = "Enter user first name", @class = "form-control ip" })

             <label style="font-weight:normal" for="#txtLName">Last name *</label>
             @Html.TextBoxFor(m => m.lname, new { id = "txtLname", style = "padding-right:20px !important;", tabindex = "4", autocomplete = "off", spellcheck = "false", placeholder = "Enter user last name", @class = "form-control ip" })
       }

     Upload image @Html.TextBoxFor(m => m.profilepic, new { type="file", id="userImageFile", tabindex=6, data_charset="file"})

     <label style="font-weight:normal" for="#txtUEmail">Email [optional]</label>
     @Html.TextBoxFor(m => m.email, new { id = "txtUEname", style = "padding-right:20px !important;", tabindex = "7", autocomplete = "off", spellcheck = "false", placeholder = "Enter user email[optional]", @class = "form-control ip" })

     <label style="font-weight:normal" for="#txtUContact">Contact [optional]</label>
     @Html.TextBoxFor(m => m.contact, new { id = "txtUContact", style = "padding-right:20px !important;", tabindex = "8", autocomplete = "off", spellcheck = "false", placeholder = "Enter user contact[optional]", @class = "form-control ip", data_maxlength="10" })

     <button class="btn btn-theme btn-block" name="add" onclick="javascript:AddUser(this);" tabindex="9" id="btnAddUser" type="submit"><i class="fa fa-save"></i> Save</button>

     <button class="btn btn-default btn-block" name="cancel" onclick="javascript: ResetAddUserForm('#frmAddUser');" tabindex="10" id="btnClear" type="button"><i class="fa fa-refresh"></i> Clear</button>

}

and here is my ajax call:

function AddUser(ctrl)
{
    var data = "";
    $("#frmAddUser").on("submit", function (e) {
        e.preventDefault();
        ValidateForm("#frmAddUser");
        var formContainer = $('#frmAddUser .text-danger');
        if ($(formContainer).text().length == 0) {
            $.ajax({
                url: '/Admin/AddUser',
                type: "POST",
                dataType:'json',
                contentType: "application/json",
                data: $('form#frmAddUser').serialize(),
                success: function (data) {
                    if (data.result) {
                        ResetForm('#frmAddUser');
                        toastr.success(data.message, "Success");
                    }
                    else {
                        toastr.error(data.message, "Failed");
                    }
                },
                error:
                function (data) {
                    toastr.error(data.message, "Failed");
                }
            });
        }

        $("#frmAddUser").unbind('submit');
        return false;
    });
}

and atlast my controller method:

[HttpPost]
  public ActionResult AddUser(AdminViewModel.AddNewUser usermodel) //->Always null
  {
       ......
  }

Waiting for any help!!

EDIT - I know this can be done using FormData instead of $('form').serialize() but I just want to try with this method!!