You either need to check all files existed at '/opt/lampp/htdocs/upl/images'
and check if the same named file is available or not, if available then just rename the file with extra "_1" every time, or you can always upload the file by renaming the file into some machine name sort of thing see the code below,
$name = rand(1000,9999) . time(); // rand(1000,9999) optional
$name = md5($name); //optional
$model->image->saveAs('/opt/lampp/htdocs/upl/images/' . $name . '.jpg');
This is what I usually do with file uploads, provided that you're saving the files references into the database or in any text file.
EDIT
Get Extension.
In case if you're required to get extension of the file rather then of hard-coded, you can use $model->image->getExtensionName();
it will get you the extension of the uploaded file without .
(dot)