title: checking file format during upload a file in grocery CRUD
body: <p>I have the following code for file upload using grocery CRUD</p>
  
  <pre><code>function test(){
          $crud=new grocery_CRUD();       
          $crud-&gt;set_table('test');
          $crud-&gt;add_fields('name', 'image');
          $crud-&gt;set_field_upload('image', 'test');
          $crud-&gt;callback_before_upload('image', array($this, 'before_test_upload'));
          $output=$crud-&gt;render();
          $output-&gt;page_title="Test Page";
          $this-&gt;_crud_output($output);       
      }
  function before_test_upload($files_to_upload, $field_info){
          // Here I want to check the file format before that file upload to source folder.
      }
  </code></pre>