Return the mime type of an extension.
$mime = File::mime_by_ext('png'); // "image/png"
string Core_File::mime_by_ext( string $extension )
参数列表
参数 类型 描述 默认值 $extensionstringExtension: php, pdf, txt, etc 
string mime type on successFALSE on failurepublic static function mime_by_ext($extension)
{
    $mimes = Core::config('mimes');
	return isset($mimes[$extension])?$mimes[$extension][0]:false;
}