php脚本以适当的扩展名保存图像

I made some javascrit code base on image uploader to save an image to the server using php. This is my code :

var reader = new FileReader();
    reader.readAsDataURL(files[0]);

    reader.onload = function(event) {
        var base64 = reader.result;     
        //console.log(base64);
        $.ajax({
            url: 'imageapi.php',
            method: 'post',
            data: {src :base64 },
            type:"POST",
            contentType:"application/x-www-form-urlencoded",
            success: function(response) {
              alert('Files uploaded successfully.');
            }
        });
    }

The uploaded file should be : text, png or svg

I want to save the file with proper extension on server. Could anyone help to solve my problem ?

I've tried using this code to get extension but it didn't work

echo $mime_type = finfo_buffer($f, $_POST['src'], FILEINFO_MIME_TYPE);

Please help me, How to save image on server with proper extension

Following code gets filename at first and then extracts it's extension.

$path = $_FILES['src']['name'];
$ext  = pathinfo($path, PATHINFO_EXTENSION);