I want to change the multipleselect extra field list of K2 Add new article, with check boxes. I did something but not perfect, similar to radiolist:in file administrator/components/com_k2/models/extrafield.php
Code:
         case 'multipleSelect':         $output=JHTML::_('select.genericlist', $defaultValues, 'K2ExtraField_'.$extraField->id.'[]', 'multiple="multiple"', 'value', 'name',$active);         break;

I changed with:
Code:
         case 'multipleSelect':         $output=JHTML::_('select.checkboxlist', $defaultValues, 'K2ExtraField_'.$extraField->id.'[]', '', 'value', 'name',$active);         break;

in file libraries/joomla/html/html/select.phpI add:
Code:
/*custom checkbox*/   function checkboxlist( $arr, $name, $attribs = null, $key = 'value', $text = 'text', $selected = null, $idtag = false, $translate = false )   {      reset( $arr );      $html = '';      if (is_array($attribs)) {         $attribs = JArrayHelper::toString($attribs);       }      $id_text = $name;      if ( $idtag ) {         $id_text = $idtag;      }      for ($i=0, $n=count( $arr ); $i < $n; $i++ )      {         $k   = $arr[$i]->$key;         $t   = $translate ? JText::_( $arr[$i]->$text ) : $arr[$i]->$text;         $id   = ( isset($arr[$i]->id) ? @$arr[$i]->id : null);         $extra   = '';         $extra   .= $id ? " id=\"" . $arr[$i]->id . "\"" : '';         if (is_array( $selected ))         {            foreach ($selected as $val)            {               $k2 = is_object( $val ) ? $val->$key : $val;               if ($k == $k2)               {                  $extra .= " selected=\"selected\"";                  break;               }            }         } else {            $extra .= ((string)$k == (string)$selected ? " checked=\"checked\"" : '');         }         $html .= "\n\t<input type=\"checkbox\" name=\"$name\" id=\"$id_text$k\" value=\"".$k."\"$extra $attribs />";         $html .= "\n\t<label for=\"$id_text$k\">$t</label><br>";      }      $html .= "\n";      return $html;   }/*end custom checkbox*/

I save the article and everything is normal, the extra field appears as I want.The only problem is when I edit the artcile, the check boxes are not selected, I have to check every option again.If someone can help me to solve this problem I'll be thankfull.

I found it:
$extra .= " selected=\"selected\"";
have to be

$extra .= " checked=\"checked\"";