PHP附件上传

Question

I have a form like this:

<form method="POST">
  <input type="text" name="title">
  <textarea rows="4" cols="50" name="description"></textarea>
  <input type="file" multiple>
  <input type="submit">
</form>

I want to upload the files when user selects them (onChange event) and get an ID. With that ID I can create inputs like:

<input type="hidden" value="ID" name="images[]">

I can upload those file with Ajax, but let's suppose that for some reason user's browser closes or his computer shutdowns before user actually click submit button. How can I delete those files from server and database ?