$(document).ready(function(){
  $('#postform').submit(function(event){
      event.preventDefault();
      console.log(event);
      $.ajax({
          url:'insertposttext.php',
          method:'POST',
          data:new FormData(this),
          contentType:false,
          processData:false,
          success:function(response){
              $("#file:input").val("");
              $("#posttext:imput").val("");
              $("#posttext:imput")[0].placeholder=response;
          }
      });
  });

});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form id="postform" enctype="multipart/form-data">
  <input type="text" name="posttext" id="posttext" placeholder="Type somthing !" class="form-control">
  <input type="file" name="file" id="file" class="form-control-file">
  <button type="submit" name="post" id="post" class="btn btn-info">Post</button>
</form>