Codeigniter Form Validation not working

PHP Code:

public function add_post()
        {
                $validation = array (
                            array(
                                'field' => 'post_title',
                                'label' => 'Post title',
                                'rules' => 'trim|required|alpha_numeric_spaces'
                                ),
                            array(
                                'field' => 'post_desc',
                                'label' => 'Post Description',
                                'rules' => 'trim|required|alpha_numeric_spaces'
                                ),
                            array(
                                'field' => 'post_content',
                                'label' => 'Post content',
                                'rules' =>  'trim|required'
                                ),
                            array(
                                'field' => 'headerimage',
                                'label' => 'File',
                                'rules' => 'required|callback_file_check'
                                )
    
                        );
    
                $this->form_validation->set_rules($validation);
                
                if($this->form_validation->run()===FALSE)
                {
                    $info['errors'] = validation_errors();
                    $info['success'] = false;
                }
                else
                {
                    $this->save_image();
                    $datetime = $this->get_time();
                    $data = array(
                                "post_title" => $this->input->post('post_title'),
                                "post_content" => $this->input->post('post_content'),
                                "post_image" => $this->uploadFileName,
                                "post_created" => $datetime
                            );
    
                    $this->Blog_model->add_post($data);
    
                    $info['success'] = true;
                    $info['message'] = "Successfully added blog post";
               }
            $this->output->set_content_type('application/json')->set_output(json_encode($info));
        }

PHP Code:

  <?php echo form_open_multipart('#',array("class"=>"form-horizontal","id"=>"blogform")); ?>
          <div class="row">
                    <div class="col-md-6">
                <input type="text" placeholder="Enter your post title" name="post_title" class="form-control">
            </div>
              <div class="col-md-6 form-group">
                <label for="file" class="control-label col-md-4">Select header image:</label>
                  <div class="col-md-8">
                  <input type="file" id="header" name="headerimage" accept="image/*" />
                </div>
            </div>
          </div>
            <input type="text" placeholder="Enter your post description" name="post_desc" class="form-control">
              
            <label class="control-label text-muted">Post Body:</label>
               <div>
                 <textarea id="post_content"></textarea>
               </div>
                <button class="btn btn-default pull-right" type="button" id="save-post"><i class="fa fa-save"></i> Save Post</button>
                <div class="clearfix"></div>
           <?php echo form_close(); ?>

Code:

$(document).on('click','#save-post',function(){

            $post_content = $('#post_content').summernote('code');

            $.ajax({

                url:site_url('Blog/add_post'),

                data: $('#blogform').serialize() + "&post_content=" + $post_content,

                type: "POST",

                dataType: 'json',

                encode: true,

                success: function(data){

                    if(!data.success){

                        if(data.errors){

                            $('#blog-message').html(data.errors).addClass('alert alert-danger');

                        }

                    }else{

                        alert(data.message);

                    }

                }

            });

    });