解决方案如下，其它框架雷同。
源代码（/system/libraries/upload.php 199 line)
$this->file_type = preg_replace("/^(.+?);.*$/", "\\1", $_FILES[$field]['type']);
修改成如下：
//Edit By Tacker
if(function_exists('mime_content_type')){
$this->file_type = preg_replace("/^(.+?);.*$/", "\\1", mime_content_type($this->file_temp));
}else{
$this->file_type = preg_replace("/^(.+?);.*$/", "\\1", $_FILES[$field]['type']);
}