How do I send file content using and how to recieved it in php

Question

This is the form.

**<form action="" id="tform">
<input type="file" id="ff" name="nm" value="ss">
<button type="submit" class="btn btn-success">Button</button>
</form>**

This is ajax code.

**<script>
$("#tform").submit(function(e) {
    e.preventDefault();
    
    let a = $("#ff").val();

    alert(a);

    $.ajax({
        url: "ajax.php",
            type: "POST",
            data:$("#tform").serialize(),
            beforeSend: function() {

               console.log("sending..");
            },

            success: function(response) {

              alert(response);


            },
    
    });
});

**

This is php code.

**<?php 
$pic = $_POST["nm"];
echo $pic;**

It always says Undefined array key "nm" but If change the type of it to text instead file it received that. File type did not get. Please help me I have been stuck here for 2 days.